test: drop underqualified identifier in unittest's 2.6 compat "polyfill"

Specifically, in supplemented unittest.TestCase.assertRegexpMatches
method. In Python 2.7's implementation, there's no re.MULTILINE
modifier at that very place, either.

Not sure what the original purpose of introducing that with c1c47f5 was.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>