discovery: fix mocked peer in unit tests

The mocked peer used here blocks on `sendToPeer`, which is not the
behavior of the `SendMessageLazy` of `lnpeer.Peer`. To reflect the
reality, we now make sure the `sendToPeer` is non-blocking in the tests.

discovery/sync_manager_test.go

@@ -28,7 +28,7 @@ func randPeer(t *testing.T, quit chan struct{}) *mockPeer {
 func peerWithPubkey(pk *btcec.PublicKey, quit chan struct{}) *mockPeer {
 	return &mockPeer{
 		pk:       pk,
-		sentMsgs: make(chan lnwire.Message),
+		sentMsgs: make(chan lnwire.Message, 1),
 		quit:     quit,
 	}
 }
@@ -483,7 +483,9 @@ func TestSyncManagerWaitUntilInitialHistoricalSync(t *testing.T) {
 		// transition it to chansSynced to ensure the remaining syncers
 		// aren't started as active.
 		if i == 0 {
-			assertSyncerStatus(t, s, syncingChans, PassiveSync)
+			assertSyncerStatus(
+				t, s, waitingQueryRangeReply, PassiveSync,
+			)
 			continue
 		}