Fixed fixtures testing failure

The failure was caused by generating the same warning from two tests.
The second time the same warning was raised it was swallowed by the
"once" simplefilter of warnings.

tests/fixtures/tests.py

@@ -142,6 +142,7 @@ class FixtureLoadingTests(DumpDataAssertMixin, TestCase):
 
         # Loading a fixture that doesn't exist emits a warning
         with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
             management.call_command('loaddata', 'unknown.json', verbosity=0,
                 commit=False)
         self.assertEqual(len(w), 1)

tests/fixtures_model_package/tests.py

@@ -102,8 +102,11 @@ class FixtureTestCase(TestCase):
         )
 
         # Load a fixture that doesn't exist
-        with warnings.catch_warnings(record=True):
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
             management.call_command("loaddata", "unknown.json", verbosity=0, commit=False)
+        self.assertEqual(len(w), 1)
+        self.assertTrue(w[0].message, "No fixture named 'unknown' found.")
 
         self.assertQuerysetEqual(
             Article.objects.all(), [