Merge remote-tracking branch 'lamby/improve-parse-changes-tests-796787' into merge

* lamby/improve-parse-changes-tests-796787:
  tests/test_parse_changes.py: Move test to ParseDscTestCase as we call with dsc_file=1
  tests/test_parse_changes.py: Test that an contentless .dsc fails correctly
  tests/test_parse_changes.py: Actually use the blank file test fixture
  tests/test_parse_changes.py: Rewrite loose and difficult-to-use-correctly assertFails
  tests/test_parse_changes.py: Actually set dsc_file=1 when testing .dsc