When no service name is specified for GSSAPI, the default service name is "mongodb".
The auth spec tests expect that the auth mechanism properties contain the service name in these cases. However, libmongoc 1.15 will not include the service name in the document returned by mongoc_uri_get_auth_mechanism_properties in this case.
CDRIVER-2637 updated libmongoc to return the service name in these cases. Once we bump to libmongoc 1.16 we can remove this special-casing from our test runner.