test_home: Change requiring pygments.css to requiring app-styles.js.

This commit changes the tests based on the fact that pygments.css
will no longer be found in the template during testing. pygemnts.css
is being compiled by webpack under app-styles and so we look for the
stubentry for app-styles instead.

Tweaked by tabbott to do a cleaner test.

zerver/tests/test_home.py

@@ -37,7 +37,8 @@ class HomeTest(ZulipTestCase):
             'Next message',
             'Search streams',
             'Welcome to Zulip',
-            'pygments.css',
+            # Verify that the app styles get included
+            'app-styles-stubentry.js',
             'var page_params',
         ]