Investigate using a Mock gRPC Channel for InstantiatingGrpcChannelProviderTest

[lqiu96]

Following the flaky test reported in here: https://github.com/googleapis/sdk-platform-java/issues/2852, we discovered that some unit tests are creating a gRPC channel. Ideally, we would like to use a mock or stub instead of a real gRPC channel.

This reduces the change for flaky behavior and also would help speed up the tests (creating a channel is ~500ms).

This ticket is to investigate whether we can use a mock or stub instead of an actual channel and implement the change.

[blakeli0]

This is a quality of life testing improvement, does not affect any functionality. Downgrade to P3 and change the type to cleanup.