Suppress "Applying schema" log noise bulk when running Complement tests (#18878)

If Synapse is under test (`SYNAPSE_LOG_TESTING` is set), we don't care
about seeing the "Applying schema" log lines at the INFO level every
time we run the tests (it's 100 lines of bulk for each homeserver).

```
synapse_main | 2025-08-29 22:34:03,453 - synapse.storage.prepare_database - 433 - INFO - main - Applying schema deltas for v73
synapse_main | 2025-08-29 22:34:03,454 - synapse.storage.prepare_database - 541 - INFO - main - Applying schema 73/01event_failed_pull_attempts.sql
synapse_main | 2025-08-29 22:34:03,463 - synapse.storage.prepare_database - 541 - INFO - main - Applying schema 73/02add_pusher_enabled.sql
synapse_main | 2025-08-29 22:34:03,473 - synapse.storage.prepare_database - 541 - INFO - main - Applying schema 73/02room_id_indexes_for_purging.sql
synapse_main | 2025-08-29 22:34:03,482 - synapse.storage.prepare_database - 541 - INFO - main - Applying schema 73/03pusher_device_id.sql
synapse_main | 2025-08-29 22:34:03,492 - synapse.storage.prepare_database - 541 - INFO - main - Applying schema 73/03users_approved_column.sql
synapse_main | 2025-08-29 22:34:03,502 - synapse.storage.prepare_database - 541 - INFO - main - Applying schema 73/04partial_join_details.sql
synapse_main | 2025-08-29 22:34:03,513 - synapse.storage.prepare_database - 541 - INFO - main - Applying schema 73/04pending_device_list_updates.sql
...
```


The Synapse logs are visible when a Complement test fails or you use
`COMPLEMENT_ALWAYS_PRINT_SERVER_LOGS=1`. This is spawning from a
Complement test with three homeservers and wanting less log noise to
scroll through.

Author: MadLittleMods

changelog.d/18878.docker

@@ -0,0 +1 @@
+Suppress "Applying schema" log noise bulk when `SYNAPSE_LOG_TESTING` is set.

docker/conf/log.config

@@ -77,6 +77,13 @@ loggers:
     #}
     synapse.visibility.filtered_event_debug:
         level: DEBUG
+
+    {#
+      If Synapse is under test, we don't care about seeing the "Applying schema" log
+      lines at the INFO level every time we run the tests (it's 100 lines of bulk)
+    #}
+    synapse.storage.prepare_database:
+      level: WARN
     {% endif %}
 
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