[bazel] Enable layering_check for MLIR test directory

This would have enabled me to notice the MLIR test file needed updating too, preventing the test file of 2074eef5 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.

Differential Revision: https://reviews.llvm.org/D118125