Move test for missing credentials to its own block

Move the test for missing credentials when we supply an authentication
token to its own block, so that:

  a) we only test on thing at a time
  b) we can mark this test as pending, which I'll do in the next commit
Mark missing credentials test as pending

This test is currently failing on `master` due to changes upstream in the
fog-core gem.

Specifically, fog/fog-core@08df305 changed the
`::Fog::Errors::LoadError` class but omitted to update a `rescue` line
that refers to that class.

I've raised a pull request against fog-core to fix that issue:
https://github.com/fog/fog-core/pull/97

In the meantime, to fix `master` in a timely manner, mark this test as
pending while we wait for a new release of `fog-core` including the fix
in that PR.
Merge pull request #145 from gds-operations/83470576-mark_test_as_pending

[#83470576] Mark missing credentials test as pending