s3tables: Add log message when cluster shutdown times out

The timeout path (2 second wait for graceful shutdown) was silent. Add a
warning log message when it occurs to help diagnose flaky test issues and
indicate when the mini cluster didn't shut down cleanly.

test/s3tables/s3tables_integration_test.go

@@ -525,6 +525,8 @@ func (c *TestCluster) Stop() {
 		// Goroutine finished
 	case <-timer.C:
 		// Timeout - goroutine doesn't respond to context cancel
+		// This may indicate the mini cluster didn't shut down cleanly
+		c.t.Log("Warning: Test cluster shutdown timed out after 2 seconds")
 	}
 
 	// Reset the global cmdMini flags to prevent state leakage to other tests