Ignore another ERROR that's expected in test.

Got a test failure in CI because of this.

test_runner/regress/test_tenant_size.py

@@ -263,6 +263,8 @@ def test_get_tenant_size_with_multiple_branches(neon_env_builder: NeonEnvBuilder
         except PageserverApiException as e:
             # compaction is ok but just retry if this fails; related to #2442
             if "cannot lock compaction critical section" in str(e):
+                # also ignore it in the log
+                env.pageserver.allowed_errors.append(".*cannot lock compaction critical section.*")
                 time.sleep(1)
                 continue
             raise