Use async anonymous credentials unconditionally in async tests

[ohmayr]

asynchronous tests should use google.auth.aio.credentials.AnonymousCredentials unconditionally once the minimum supported version of google-auth is upgraded to one which exposes google.auth.aio.

As of now, async tests will either use google.auth.aio.credentials.AnonymousCredentials if available or otherwise default to google.auth.credentials.AnonymousCredentials.