tests: refactor mock_get() to a recorder
tests: refactor mock_get() to a recorder

Refactor the mock_get() helper method into a true PyTest fixture that
returns a callable object. This object will record the kwargs.

The purpose of this change is to support a new mock_post() fixture (in a
future commit).

ktdreyer/errata-tool,mmuzila/errata-tool,mmuzila/errata-tool,red-hat-storage/errata-tool,red-hat-storage/errata-tool,ktdreyer/errata-tool