tests and mocks for e2e testing (#78)

* tests and mocks for e2e testing

* formatting

* fix analyze errors

example/lib/widgets/participant.dart

@@ -158,6 +158,7 @@ class _LocalParticipantWidgetState
         _visible) {
       return firstVideoPublication?.track;
     }
+    return null;
   }
 }
 
@@ -180,6 +181,7 @@ class _RemoteParticipantWidgetState
         return trackPublication.track;
       }
     }
+    return null;
   }
 
   @override

example/pubspec.lock

@@ -240,6 +240,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "0.12.11"
+  material_color_utilities:
+    dependency: transitive
+    description:
+      name: material_color_utilities
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.1.3"
   meta:
     dependency: transitive
     description:
@@ -475,7 +482,7 @@ packages:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.4.3"
+    version: "0.4.8"
   typed_data:
     dependency: transitive
     description: