device: move stats fields back down and add test diagnostics

This reverts the movement of fields from d49f4e9.
That commit was cherry-picked from another branch where a field
had changed and misaligned the atomic fields. After cherry-picking,
moving the fields was no longer necessary but got dragged along.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

device/peer.go

@@ -21,6 +21,14 @@ const (
 )
 
 type Peer struct {
+	// Mostly protects endpoint, but is generally taken whenever we modify peer
+	sync.RWMutex
+	keypairs                    Keypairs
+	handshake                   Handshake
+	device                      *Device
+	endpoint                    conn.Endpoint
+	persistentKeepaliveInterval uint16
+
 	// These fields are accessed with atomic operations, which must be
 	// 64-bit aligned even on 32-bit platforms. Go guarantees that an
 	// allocated struct will be 64-bit aligned. So we place
@@ -35,14 +43,6 @@ type Peer struct {
 	// bits. Don't place other atomic fields after this one.
 	isRunning AtomicBool
 
-	// Mostly protects endpoint, but is generally taken whenever we modify peer
-	sync.RWMutex
-	keypairs                    Keypairs
-	handshake                   Handshake
-	device                      *Device
-	endpoint                    conn.Endpoint
-	persistentKeepaliveInterval uint16
-
 	timers struct {
 		retransmitHandshake     *Timer
 		sendKeepalive           *Timer