GnomeKeyring: fix the globals check to make it actually working
GnomeKeyring: more availablility checks fixes

* Call enumerate_versions() before actually importing the module,
  it was needed to make sure pygobject doesn't print warning messages when importing.
* Raise RuntimeError when requisite environment vars are not present.
Fixes to GnomeKeyring and SecretService tests

* Keyrings no longer have `supported()` method.
* s/gnomekeyring/gi.repository/ in GnomeKeyring tests.
* SecretService test's debug messages no longer pollute tests output.
* Drop `test_supported_no_dbus` check, we don't rely on dbus directly.

--HG--
extra : amend_source : 1284ee5a3a09f3c196f9345c94bfb4adaa98916b