Properly call setup_background_tasks in unit tests. (#16150)

This should only be called on HomeServer objects which are configured
to run background tasks, which is automatically (and properly) done via
the call to setup().

changelog.d/16150.misc

@@ -0,0 +1 @@
+Clean-up calling `setup_background_tasks` in unit tests.

tests/server.py

@@ -1000,8 +1000,6 @@ def setup_test_homeserver(
     hs.tls_server_context_factory = Mock()
 
     hs.setup()
-    if homeserver_to_use == TestHomeServer:
-        hs.setup_background_tasks()
 
     if isinstance(db_engine, PostgresEngine):
         database_pool = hs.get_datastores().databases[0]