Turn on `MTL_DEBUG_LAYER_WARNING_MODE` for Api Validation tests

[christiangnrd]

From man MetalValidation:

MTL_DEBUG_LAYER_WARNING_MODE
      Defaults to "ignore"
      Valid values are "assert", "ignore" and "nslog"
      Sets the behavior for when a debug layer warning occurs.

      Setting "assert" will cause the debug layer to log and then assert on warnings.

      Setting "ignore" will cause the debug layer to ignore warnings.

      Setting "nslog" will cause the debug layer to log warnings using NSLog.

Trying it at the moment will flood the terminal with warnings.

[christiangnrd]

This is probably not feasible since MPS APIs trigger warnings