tests: extend allow list in deletion test (#8268)

## Problem

1ea5d8b tolerated this as an error
message, but it can show up in logs as well.

Example failure:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8201/9780147712/index.html#testresult/263422f5f5f292ea/retries

## Summary of changes

- Tolerate "failed to delete 1 objects" in pageserver logs, this occurs
occasionally when injected failures exhaust deletion's retries.

test_runner/regress/test_tenant_delete.py

@@ -67,8 +67,9 @@ def test_tenant_delete_smoke(
 
     # first try to delete non existing tenant
     tenant_id = TenantId.generate()
-    env.pageserver.allowed_errors.append(".*NotFound.*")
-    env.pageserver.allowed_errors.append(".*simulated failure.*")
+    env.pageserver.allowed_errors.extend(
+        [".*NotFound.*", ".*simulated failure.*", ".*failed to delete .+ objects.*"]
+    )
 
     # Check that deleting a non-existent tenant gives the expected result: this is a loop because we
     # may need to retry on some remote storage errors injected by the test harness