tests: Add 100% test coverage to zerver/apps.py.

Tweaked by tabbott to rename to test_cache.py and remove the
sender_name argument (Since it was kinda confusing).

zerver/tests/test_cache.py

@@ -0,0 +1,13 @@
+from mock import Mock, patch
+
+from zerver.apps import flush_cache
+from zerver.lib.test_classes import ZulipTestCase
+
+class AppsTest(ZulipTestCase):
+    def test_cache_gets_flushed(self) -> None:
+        with patch('zerver.apps.logging.info') as mock_logging:
+            with patch('zerver.apps.cache.clear') as mock:
+                # The argument to flush_cache doesn't matter
+                flush_cache(Mock())
+                mock.assert_called_once()
+            mock_logging.assert_called_once()