[bazel] Enable layering_check for MLIR test directory

[rupprecht]

Summary

This would have enabled me to notice the MLIR test file needed updating too, preventing the test file of 2074eef5db330ab437410bfb617b58ea70f4fbff from being necessary.

layering_check is already enabled in mlir/BUILD.bazel. I don't know why I didn't see the other breakage there.

[GMNGeoffrey]

LGTM, thanks

utils/bazel/llvm-project-overlay/mlir/test/BUILD.bazel

 # This file is licensed under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 load("//mlir:tblgen.bzl", "gentbl_cc_library", "td_library")
 
 package(
     default_visibility = ["//visibility:public"],
+    features = ["layering_check"],
     licenses = ["notice"],
 )
 
 cc_library(
     name = "IRProducingAPITest",
     hdrs = ["APITest.h"],
     includes = ["."],
 )