Add tests for empty protocols on built-in objects.

[thetorpedodog]

One of the symptoms of the earlier problem was that you could not check protocols against built-in types at all. This adds a test specifically for that situation to prevent a regression.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• [n/a] You've added tests (in tests/) added which would fail without your patch
  Regression test for change Use get_protocol_members in protocol checking #490.
• [n/a] You've updated the documentation (in docs/, in case of behavior changes or new features)
• [n/a] You've added a new changelog entry (in docs/versionhistory.rst).

[coveralls]

coverage: 94.217%. remained the same
when pulling e217aba on thetorpedodog:test-empty-protocol
into cf25d56 on agronholm:master.