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 @@ def test_loading_and_dumping(self):
 
         # 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 @@ def test_loaddata(self):
         )
 
         # 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(), [