tests: tweak log allow list in test_sharding_split_failures (#7549)

## Problem

This test became flaky recently with failures like:
```
AssertionError: Log errors on storage_controller: (129, '2024-04-29T16:41:03.591506Z ERROR request{method=PUT path=/control/v1/tenant/b38c0447fbdbcf4e1c023f00b0f7c221/shard_split request_id=34df4975-2ef3-4ed8-b167-2956650e365c}: Error processing HTTP request: InternalServerError(Reconcile error on shard b38c0447fbdbcf4e1c023f00b0f7c221-0002: Cancelled\n')
```

Likely due to #7508 changing how errors are reported from Reconcilers.

## Summary of changes

- Tolerate `Reconcile error.*Cancelled` log errors

test_runner/regress/test_sharding.py

@@ -928,6 +928,8 @@ def test_sharding_split_failures(
             ".*Reconcile error: receive body: error sending request for url.*",
             # Node offline cases will fail inside reconciler when detaching secondaries
             ".*Reconcile error on shard.*: receive body: error sending request for url.*",
+            # Node offline cases may eventually cancel reconcilers when the heartbeater realizes nodes are offline
+            ".*Reconcile error.*Cancelled.*",
             # While parent shard's client is stopped during split, flush loop updating LSNs will emit this warning
             ".*Failed to schedule metadata upload after updating disk_consistent_lsn.*",
         ]