tests: allow a log warning in test_cli_start_stop_multi (#9320)

## Problem

This test restarts services in an undefined order (whatever neon_local
does), which means we should be tolerant of warnings that come from
restarting the storage controller while a pageserver is running.

We can see failures with warnings from dropped requests, e.g.
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9307/11229000712/index.html#/testresult/d33d5cb206331e28
```
 WARN request{method=GET path=/v1/location_config request_id=b7dbda15-6efb-4610-8b19-a3772b65455f}: request was dropped before completing\n')
```

## Summary of changes

- allow-list the `request was dropped before completing` message on
pageservers before restarting services

test_runner/regress/test_neon_cli.py

@@ -162,6 +162,11 @@ def test_cli_start_stop_multi(neon_env_builder: NeonEnvBuilder):
     env.neon_cli.pageserver_stop(env.BASE_PAGESERVER_ID)
     env.neon_cli.pageserver_stop(env.BASE_PAGESERVER_ID + 1)
 
+    # We will stop the storage controller while it may have requests in
+    # flight, and the pageserver complains when requests are abandoned.
+    for ps in env.pageservers:
+        ps.allowed_errors.append(".*request was dropped before completing.*")
+
     # Keep NeonEnv state up to date, it usually owns starting/stopping services
     env.pageservers[0].running = False
     env.pageservers[1].running = False