Be able to shutdown homeserver that hasn't setup (#19187)

For example, a homeserver can fail to `setup` if it fails to connect to
the database.

Fix https://github.com/element-hq/synapse/issues/19188

Follow-up to https://github.com/element-hq/synapse/pull/18828


### Background

As part of Element's plan to support a light form of vhosting (virtual
host) (multiple instances of Synapse in the same Python process) (c.f
Synapse Pro for small hosts), we're currently diving into the details
and implications of running multiple instances of Synapse in the same
Python process.

"Clean tenant deprovisioning" tracked internally by
https://github.com/element-hq/synapse-small-hosts/issues/50

tests/crypto/test_keyring.py

@@ -95,7 +95,12 @@ class KeyringTestCase(unittest.HomeserverTestCase):
     def test_verify_json_objects_for_server_awaits_previous_requests(self) -> None:
         mock_fetcher = Mock()
         mock_fetcher.get_keys = Mock()
-        kr = keyring.Keyring(self.hs, key_fetchers=(mock_fetcher,))
+        kr = keyring.Keyring(
+            self.hs,
+            test_only_key_fetchers=[
+                mock_fetcher,
+            ],
+        )
 
         # a signed object that we are going to try to validate
         key1 = signedjson.key.generate_signing_key("1")
@@ -286,7 +291,7 @@ class KeyringTestCase(unittest.HomeserverTestCase):
         mock_fetcher = Mock()
         mock_fetcher.get_keys = Mock(side_effect=get_keys)
         kr = keyring.Keyring(
-            self.hs, key_fetchers=(StoreKeyFetcher(self.hs), mock_fetcher)
+            self.hs, test_only_key_fetchers=[StoreKeyFetcher(self.hs), mock_fetcher]
         )
 
         # sign the json
@@ -313,7 +318,7 @@ class KeyringTestCase(unittest.HomeserverTestCase):
 
         mock_fetcher = Mock()
         mock_fetcher.get_keys = Mock(side_effect=get_keys)
-        kr = keyring.Keyring(self.hs, key_fetchers=(mock_fetcher,))
+        kr = keyring.Keyring(self.hs, test_only_key_fetchers=[mock_fetcher])
 
         json1: JsonDict = {}
         signedjson.sign.sign_json(json1, "server1", key1)
@@ -363,7 +368,9 @@ class KeyringTestCase(unittest.HomeserverTestCase):
         mock_fetcher1.get_keys = Mock(side_effect=get_keys1)
         mock_fetcher2 = Mock()
         mock_fetcher2.get_keys = Mock(side_effect=get_keys2)
-        kr = keyring.Keyring(self.hs, key_fetchers=(mock_fetcher1, mock_fetcher2))
+        kr = keyring.Keyring(
+            self.hs, test_only_key_fetchers=[mock_fetcher1, mock_fetcher2]
+        )
 
         json1: JsonDict = {}
         signedjson.sign.sign_json(json1, "server1", key1)