test_lsn_lease_storcon: ignore ShardSplit warning in debug builds (#12770)

## Problem

`test_lsn_lease_storcon` might fail in debug builds due to slow
ShardSplit

## Summary of changes
- Make `test_lsn_lease_storcon ` test to ignore `.*Exclusive lock by
ShardSplit was held.*` warning in debug builds

Ref: https://databricks.slack.com/archives/C09254R641L/p1753777051481029

test_runner/regress/test_tenant_size.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import os
 from concurrent.futures import ThreadPoolExecutor
 from typing import TYPE_CHECKING
 
@@ -768,6 +769,14 @@ def test_lsn_lease_storcon(neon_env_builder: NeonEnvBuilder):
         "compaction_period": "0s",
     }
     env = neon_env_builder.init_start(initial_tenant_conf=conf)
+    # ShardSplit is slow in debug builds, so ignore the warning
+    if os.getenv("BUILD_TYPE", "debug") == "debug":
+        env.storage_controller.allowed_errors.extend(
+            [
+                ".*Exclusive lock by ShardSplit was held.*",
+            ]
+        )
+
     with env.endpoints.create_start(
         "main",
     ) as ep: