Tests to ensure proper direction in SDP

Per @Sean-Der's comments in this PR
(https://github.com/pion/webrtc/pull/3198), adding a couple of more
tests to ensure that direction in SDP is correct when remote offer is
processed and transceivers are matched up against existing ones using
type and direction.

Test flow
- AddTrack first on answer side. This should create a transceiver with
  `sendrecv`
- Receive remote description offer
- First time, it will try to match using type and direction.
  o If the remote offer had a media section with `sendonly`, the above
    should fail to find a match. In this case, a new transceiver will
    be created. So, there should be 2 transceivers (one from this +
    one from AddTrack in Step 1). In this case, the answer will have
    only one media section as the offer had only one slot. And the
    directions should be `remote = sendonly` and `local = recvonly`.
  o If the remote offer is `sendrecv`, it should find a match. So, there
    should be only one transceiver (the one created with AddTrack gets
    used for receive).  In this case, the answer will have
    only one media section as the offer had only one slot. And the
    directions should be `remote = sendrecv` and `local = sendrecv`.
- Then the remote track is removed from offer. Processing that remote
  decription will find a transceiver by `mid`, but directions are
  checked in the SDP to ensure they have correct values.

peerconnection_go_test.go

@@ -1285,6 +1285,146 @@ func TestPeerConnection_TransceiverDirection(t *testing.T) {
 	}
 }
 
+func TestPeerConnection_MediaDirectionInSDP(t *testing.T) {
+	lim := test.TimeOut(time.Second * 30)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	createTransceiver := func(pc *PeerConnection, dir RTPTransceiverDirection) (*RTPSender, error) {
+		// AddTransceiverFromKind() can't be used with sendonly
+		if dir == RTPTransceiverDirectionSendonly {
+			codecs := pc.api.mediaEngine.getCodecsByKind(RTPCodecTypeVideo)
+
+			track, err := NewTrackLocalStaticSample(codecs[0].RTPCodecCapability, util.MathRandAlpha(16), util.MathRandAlpha(16))
+			if err != nil {
+				return nil, err
+			}
+
+			transceiver, err := pc.AddTransceiverFromTrack(track, []RTPTransceiverInit{
+				{Direction: dir},
+			}...)
+
+			return transceiver.Sender(), err
+		}
+
+		transceiver, err := pc.AddTransceiverFromKind(
+			RTPCodecTypeVideo,
+			RTPTransceiverInit{Direction: dir},
+		)
+
+		return transceiver.Sender(), err
+	}
+
+	testCases := []struct {
+		remoteDirections         []RTPTransceiverDirection
+		numExpectedTransceivers  int
+		numExpectedMediaSections int
+		localDirections          []RTPTransceiverDirection
+	}{
+		{
+			remoteDirections: []RTPTransceiverDirection{
+				RTPTransceiverDirectionSendonly,
+				RTPTransceiverDirectionInactive,
+			},
+			numExpectedTransceivers:  2,
+			numExpectedMediaSections: 1,
+			localDirections: []RTPTransceiverDirection{
+				RTPTransceiverDirectionRecvonly,
+				RTPTransceiverDirectionInactive,
+			},
+		},
+		{
+			remoteDirections: []RTPTransceiverDirection{
+				RTPTransceiverDirectionSendrecv,
+				RTPTransceiverDirectionRecvonly,
+			},
+			numExpectedTransceivers:  1,
+			numExpectedMediaSections: 1,
+			localDirections: []RTPTransceiverDirection{
+				RTPTransceiverDirectionSendrecv,
+				RTPTransceiverDirectionSendonly,
+			},
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run("add track before remote description - "+testCase.remoteDirections[0].String(), func(t *testing.T) {
+			pcOffer, pcAnswer, err := newPair()
+			assert.NoError(t, err)
+
+			// add track to answerer before any remote description, added transceiver will be `sendrecv`
+			track, err := NewTrackLocalStaticSample(RTPCodecCapability{MimeType: MimeTypeVP8}, "foo", "bar")
+			assert.NoError(t, err)
+			_, err = pcAnswer.AddTrack(track)
+			assert.NoError(t, err)
+
+			sender, err := createTransceiver(pcOffer, testCase.remoteDirections[0])
+			assert.NoError(t, err)
+
+			offer, err := pcOffer.CreateOffer(nil)
+			assert.NoError(t, err)
+			assert.NoError(t, pcOffer.SetLocalDescription(offer))
+
+			// transceiver created from remote description
+			//  - cannot match track added above if remote direction is `sendonly`
+			//  - can match track added above if remote direction is `sendrecv`
+			assert.NoError(t, pcAnswer.SetRemoteDescription(offer))
+			assert.Equal(t, testCase.numExpectedTransceivers, len(pcAnswer.GetTransceivers()))
+
+			answer, err := pcAnswer.CreateAnswer(nil)
+			assert.NoError(t, err)
+
+			// direction has to be `recvonly` in answer if remote direction is `sendonly`
+			// direction has to be `sendrecv` in answer if remote direction is `sendrecv`
+			parsed, err := answer.Unmarshal()
+			assert.NoError(t, err)
+			assert.Equal(t, testCase.numExpectedMediaSections, len(parsed.MediaDescriptions))
+			_, ok := parsed.MediaDescriptions[0].Attribute(testCase.localDirections[0].String())
+			assert.True(t, ok)
+
+			assert.NoError(t, pcAnswer.SetLocalDescription(answer))
+			assert.NoError(t, pcOffer.SetRemoteDescription(answer))
+
+			// remove the remote track and re-negotiate
+			//  - both directions should become `inactive` if original remote direction was `sendonly`
+			//  - remote direction should become `recvonly and local direction should become `sendonly`
+			//    if original remote direction was `sendrecv`
+			assert.NoError(t, pcOffer.RemoveTrack(sender))
+
+			offer, err = pcOffer.CreateOffer(nil)
+			assert.NoError(t, err)
+
+			// offer direction should have changed to the following after removing track
+			//   - `inactive` if original offer direction was `sendonly`
+			//   - `recvonly` if original offer direction was `sendrecv`
+			parsed, err = offer.Unmarshal()
+			assert.NoError(t, err)
+			assert.Equal(t, testCase.numExpectedMediaSections, len(parsed.MediaDescriptions))
+			_, ok = parsed.MediaDescriptions[0].Attribute(testCase.remoteDirections[1].String())
+			assert.True(t, ok)
+
+			assert.NoError(t, pcOffer.SetLocalDescription(offer))
+			assert.NoError(t, pcAnswer.SetRemoteDescription(offer))
+
+			answer, err = pcAnswer.CreateAnswer(nil)
+			assert.NoError(t, err)
+
+			// answer direction should have changed to
+			//   - `inactive` if original offer direction was `sendonly`
+			//   - `sendonly` if original offer direction was `sendrecv`
+			parsed, err = answer.Unmarshal()
+			assert.NoError(t, err)
+			assert.Equal(t, testCase.numExpectedMediaSections, len(parsed.MediaDescriptions))
+			_, ok = parsed.MediaDescriptions[0].Attribute(testCase.localDirections[1].String())
+			assert.True(t, ok)
+
+			closePairNow(t, pcOffer, pcAnswer)
+		})
+	}
+}
+
 func TestPeerConnection_SessionID(t *testing.T) {
 	defer test.TimeOut(time.Second * 10).Stop()
 	defer test.CheckRoutines(t)()