Upgrade to test package version 0.12.30+4 (#14853)

30+4 has made some changes to the remoting protocol that are incompatbile
with a test harness running 30+3 (e.g. adding a type='initial' field to the
first message)
diff --git a/examples/flutter_gallery/pubspec.yaml b/examples/flutter_gallery/pubspec.yaml
index de45c37..399f00a 100644
--- a/examples/flutter_gallery/pubspec.yaml
+++ b/examples/flutter_gallery/pubspec.yaml
@@ -66,7 +66,7 @@
   stack_trace: 1.9.1 # TRANSITIVE DEPENDENCY
   stream_channel: 1.6.3 # TRANSITIVE DEPENDENCY
   term_glyph: 1.0.0 # TRANSITIVE DEPENDENCY
-  test: 0.12.30+3 # TRANSITIVE DEPENDENCY
+  test: 0.12.30+4 # TRANSITIVE DEPENDENCY
   typed_data: 1.1.5 # TRANSITIVE DEPENDENCY
   utf: 0.9.0+4 # TRANSITIVE DEPENDENCY
   vector_math: 2.0.5 # TRANSITIVE DEPENDENCY