[Impeller] Metal validation failures on CanRenderTiledTexture/Metal.

[chinmaygarde]

The following validation error is reported.

[ RUN      ] Play/AiksTest.CanRenderTiledTexture/Metal
-[MTLDebugBlitCommandEncoder generateMipmapsForTexture:]:1109:
  failed assertion
    `Generate Mipmaps For Texture Validation [tex mipmapLevelCount](1) must be > 1.'

The steps to enable Metal validation in command line apps (like the Google Test harness) are here.

[chinmaygarde]

cc @bdero

[bdero]

FYI @ColdPaleLight

[github-actions]

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