[google_maps_flutter] Update latest test code

[bbrto21]

   Why don't you update the test code (and etc.) to the latest?

https://github.com/flutter/plugins/blob/main/packages/google_maps_flutter/google_maps_flutter/example/integration_test/google_maps_test.dart

Originally posted by @swift-kim in #464 (comment)

[swift-kim]

Updating the integration test code seems a little bit tricky because of the recently added test inspector interface (flutter/plugins#6133). I'm not sure how to deal with the new interface.

We also need to update the Dart implementation (lib/src) and update dependent package versions (google_maps_flutter and google_maps_flutter_platform_interface) to the latest. Could anybody help?