Adding coverage for verify_signed_jwt_with_certs.
Adding coverage for verify_signed_jwt_with_certs.

This already exists in test_jwt but these tests just make sure
the correct exceptions occur on error and make sure that in the
success case all the correct verify/check methods are called.
Doesn't actually call these methods, just uses mocks instead.

jonparrott/oauth2client,jonparrott/oauth2client,googleapis/oauth2client,clancychilds/oauth2client,googleapis/oauth2client,clancychilds/oauth2client,google/oauth2client,google/oauth2client