tests and mocks for e2e testing (#78)

* tests and mocks for e2e testing

* formatting

* fix analyze errors

lib/src/participant/local.dart

@@ -305,6 +305,7 @@ class LocalParticipant extends Participant<LocalTrackPublication> {
         return await publishVideoTrack(track);
       }
     }
+    return null;
   }
 
   /// Control who can subscribe to LocalParticipant's published tracks.

lib/src/participant/remote.dart

@@ -65,6 +65,7 @@ class RemoteParticipant extends Participant<RemoteTrackPublication> {
   RemoteTrackPublication? getTrackPublication(String sid) {
     final pub = trackPublications[sid];
     if (pub is RemoteTrackPublication) return pub;
+    return null;
   }
 
   /// for internal use