Apple7 some tests fail due to Depth16unorm being unsupported

[cwfitzgerald]

Discovered in #8610 (comment)

       ABORT [   1.401s] wgpu-test::wgpu-gpu [Executed] [Metal/Apple A7 GPU/0] wgpu_gpu::zero_init_texture_after_discard::discarding_depth_target_resets_texture_init_state_check_visible_on_copy_in_same_encoder
           - with code 0xffffffff: OS Error -268435457 (FormatMessageW() returned error 317) (os error -1)
  stdout ───

    running 1 test
  stderr ───
    validateMTLPixelFormat:174: failed assertion `pixelFormat (250) is not a valid MTLPixelFormat.'

    (test aborted)

This seems to be https://developer.apple.com/documentation/metal/mtlpixelformat/depth16unorm

[inner-daemons]

@cwfitzgerald This should be labelled as "depth16unorm". There is no such thing as depth16float in metal or wgpu.