pageserver: fix flake in test_timeline_deletion_with_files_stuck_in_u…

…pload_queue (#5149)

## Problem

Test failing on a different ERROR log than it anticipated.

Closes: #5148

## Summary of changes

Add the "could not flush frozen layer" error log to the permitted
errors.

test_runner/regress/test_remote_storage.py

@@ -604,6 +604,7 @@ def assert_compacted_and_uploads_queued():
     checkpoint_allowed_to_fail.set()
     env.pageserver.allowed_errors.append(
         ".* ERROR .*Error processing HTTP request: InternalServerError\\(timeline is Stopping"
+        ".* ERROR .*[Cc]ould not flush frozen layer.*"
     )
 
     # Generous timeout, because currently deletions can get blocked waiting for compaction