Added tests for lastSent and lastReceived get/set

Validate the new behaviour when these atomic vars have not been
set yet.

candidate_test.go

@@ -1,6 +1,11 @@
 package ice
 
-import "testing"
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
 
 func TestCandidatePriority(t *testing.T) {
 	for _, test := range []struct {
@@ -49,3 +54,19 @@ func TestCandidatePriority(t *testing.T) {
 		}
 	}
 }
+
+func TestCandidateLastSent(t *testing.T) {
+	candidate := candidateBase{}
+	assert.Equal(t, candidate.LastSent(), time.Time{})
+	now := time.Now()
+	candidate.setLastSent(now)
+	assert.Equal(t, candidate.LastSent(), now)
+}
+
+func TestCandidateLastReceived(t *testing.T) {
+	candidate := candidateBase{}
+	assert.Equal(t, candidate.LastReceived(), time.Time{})
+	now := time.Now()
+	candidate.setLastReceived(now)
+	assert.Equal(t, candidate.LastReceived(), now)
+}