Set a logging context while running the bg updates

This mostly just reduces the amount of "running from sentinel context" spam
during unittest setup.

tests/unittest.py

@@ -40,6 +40,7 @@ from synapse.http.server import JsonResource
 from synapse.http.site import SynapseRequest, SynapseSite
 from synapse.logging.context import (
     SENTINEL_CONTEXT,
+    LoggingContext,
     current_context,
     set_current_context,
 )
@@ -419,15 +420,17 @@ class HomeserverTestCase(TestCase):
         config_obj.parse_config_dict(config, "", "")
         kwargs["config"] = config_obj
 
+        async def run_bg_updates():
+            with LoggingContext("run_bg_updates", request="run_bg_updates-1"):
+                while not await stor.db.updates.has_completed_background_updates():
+                    await stor.db.updates.do_next_background_update(1)
+
         hs = setup_test_homeserver(self.addCleanup, *args, **kwargs)
         stor = hs.get_datastore()
 
         # Run the database background updates, when running against "master".
         if hs.__class__.__name__ == "TestHomeServer":
-            while not self.get_success(
-                stor.db.updates.has_completed_background_updates()
-            ):
-                self.get_success(stor.db.updates.do_next_background_update(1))
+            self.get_success(run_bg_updates())
 
         return hs