Fix an erroneous test.
Fix an erroneous test.

Since test_gce_metadata_does_not_load_when_fetch_gce_metadata_is_false
should pass in a non-GCE environment, we should not call
setup_gce_metadata_stubs in the test.  I verified this test fails
with an unmodified plugin as expected.

cosmo0920/fluent-plugin-google-cloud,swalkowski/fluent-plugin-google-cloud,mururu/fluent-plugin-google-cloud,hakobera/fluent-plugin-google-cloud,GoogleCloudPlatform/fluent-plugin-google-cloud,senthilkumar-kj/fluent-plugin-google-cloud,a-robinson/fluent-plugin-google-cloud,arthurdarcet/fluent-plugin-google-cloud,laramiel/fluent-plugin-google-cloud,brint/fluent-plugin-google-cloud