Auth tests lacking

[chrisdunelm]

More tests for ServiceAccountCredential (and probably other auth classes) needed.
e.g. ServiceAccountCredential.Initializer.FromCertificate() isn't tested at all.
e.g. ServiceAccountCredential.Initializer.FromPrivateKey() test doesn't check that the credential is valid.
etc...

[chrisdunelm]

We now have integration tests that test Service and User credentials. It's not perfect, but much improved; so closing.