set worker_app for frontend proxy test (#7003)

to stop the federationhandler trying to do master stuff

changelog.d/7003.misc

@@ -0,0 +1 @@
+Refactoring work in preparation for changing the event redaction algorithm.

tests/app/test_frontend_proxy.py

@@ -27,6 +27,11 @@ class FrontendProxyTests(HomeserverTestCase):
 
         return hs
 
+    def default_config(self, name="test"):
+        c = super().default_config(name)
+        c["worker_app"] = "synapse.app.frontend_proxy"
+        return c
+
     def test_listen_http_with_presence_enabled(self):
         """
         When presence is on, the stub servlet will not register.