Clean up push_floating_tags unit tests
Clean up push_floating_tags unit tests

* don't test multiple registries, it is no longer supported
* remove "worker" terminology
* remove unused annotations param from mock_environment()
* rename expected_exception to expected_skip_reason, because that's what
  it is (the plugin doesn't raise an exception, just returns None)
* address pylint suggestion to use dict.items() when creating the
  registries_list (and refactor to create it with single expression)

Much like the test suite itself, the changes are basically a copy-paste
from test_group_manifest.

Signed-off-by: Adam Cmiel <1217f9865bd733d1bcad0d0d64be310272c5592c@redhat.com>

fr34k8/atomic-reactor,projectatomic/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor