Remove duplicate timestamp from test logs (_trial_temp/test.log) (#…

…15636)

Fix #15618

### Before

```
2023-05-17 22:51:36-0500 [-] 2023-05-17 22:51:36,889 - synapse.server - 338 - INFO - sentinel - Finished setting up.
```

### After

```
2023-05-19 18:16:20-0500 [-] synapse.server - 338 - INFO - sentinel - Finished setting up.
```


### Dev notes

The `Twisted.Logger` controls the `2023-05-19 18:16:20-0500 [-]` prefix, see : [`twisted/twisted` -> `src/twisted/logger/_format.py#L362-L374`](https://github.com/twisted/twisted/blob/34b161e66bc7c9f9efbb95e82c770a863933e498/src/twisted/logger/_format.py#L362-L374)

And we delegate our logs to the Twisted Logger for the tests which puts it in `_trial_temp/test.log`

changelog.d/15636.misc

@@ -0,0 +1 @@
+Remove duplicate timestamp from test logs (`_trial_temp/test.log`).

tests/test_utils/logging_setup.py

@@ -40,10 +40,9 @@ def setup_logging() -> None:
     """
     root_logger = logging.getLogger()
 
-    log_format = (
-        "%(asctime)s - %(name)s - %(lineno)d - "
-        "%(levelname)s - %(request)s - %(message)s"
-    )
+    # We exclude `%(asctime)s` from this format because the Twisted logger adds its own
+    # timestamp
+    log_format = "%(name)s - %(lineno)d - " "%(levelname)s - %(request)s - %(message)s"
 
     handler = ToTwistedHandler()
     formatter = logging.Formatter(log_format)