Make add_labels_in_df unit tests more consistent
Make add_labels_in_df unit tests more consistent

Define a mock_env() function instead of having the same setup code
copy-pasted in every unit test.

Get rid of the MockSource class.

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

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