tests: give mock homeservers a real string hostname

The Rust `LoggingContext` requires `server_name` to be a `str`. Three test
suites built a mock homeserver whose `hostname` was left as an auto-generated
`Mock`, which flowed into `server_name` (via `DatabasePool`, `StateHandler`
and `ApplicationServicesHandler`) and raised `TypeError: argument
'server_name': 'Mock' object is not an instance of 'str'` once a
`LoggingContext`/`Measure` was constructed — 42 trial errors in CI.

Set `hostname` to the server name each suite already uses, so the mocks match
what production passes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VF56cZ93AqpuGCf8yguCcR

tests/handlers/test_appservice.py

@@ -77,6 +77,7 @@ class AppServiceHandlerTestCase(unittest.TestCase):
         self.reactor, self.clock = get_clock()
 
         hs = Mock()
+        hs.hostname = "test_server"
 
         def test_run_as_background_process(
             desc: "LiteralString",

tests/storage/test_base.py

@@ -105,7 +105,7 @@ class SQLBaseStoreTestCase(unittest.TestCase):
         # To fix isinstance(...) checks.
         fake_engine.__class__ = engine.__class__  # type: ignore[assignment]
 
-        db = DatabasePool(Mock(), Mock(config=db_config), fake_engine)
+        db = DatabasePool(Mock(hostname="test"), Mock(config=db_config), fake_engine)
         db._db_pool = conn_pool
 
         self.datastore = SQLBaseStore(db, None, hs)  # type: ignore[arg-type]

tests/test_state.py

@@ -245,6 +245,7 @@ class StateTestCase(unittest.TestCase):
         )
         reactor, clock = get_clock()
         hs.config = default_config(server_name="tesths", parse=True)
+        hs.hostname = "tesths"
         hs.get_datastores.return_value = Mock(
             main=self.dummy_store,
             state_deletion=dummy_deletion_store,