[1.5.x] Fixed #18985 -- ensure module level deprecations are displayed

Also don't compete with -W CLI option.

Thanks to Aymeric Augustin for the catch, and Claude Paroz for the patch.

Backport of e79b857a07 from master.

tests/regressiontests/test_runner/deprecation_app/tests.py

@@ -2,6 +2,8 @@ import warnings
 
 from django.test import TestCase
 
+warnings.warn("module-level warning from deprecation_app", DeprecationWarning)
+
 class DummyTest(TestCase):
     def test_warn(self):
         warnings.warn("warning from test", DeprecationWarning)