Metal compute tests

[skallweitNV]

This PR adds support for running compute tests with the new metal backend.

The current state is:

• 280 tests pass
• 18 tests fail (generate unexpected result)
• 155 tests fail to compile (in the metal downstream compiler)

All tests that fail to compile are disabled using

//DISABLE_TEST(compute):COMPARE_COMPUTE:-slang -shaderobj -mtl

See #4291 for more details.

[skallweitNV]

I don't quite understand why this change leads to the following failing tests:

• tests/compute/matrix-layout-structured-buffer.slang.3 syn (dx12)
• tests/compute/matrix-layout-structured-buffer.slang.4 syn (dx12)
• tests/compute/buffer-layout.slang.5 syn (cuda)

Also there is one COMPARE_HLSL_RENDER test that fails on metal:

• tests/bugs/texture2d-gather.hlsl

@csyonghe ?

[csyonghe]

The texture2d test may be using a vertex-fragment shader and we seem to be generating invalid vertex shader code for metal, causing the test to fail.

[csyonghe]

Not sure about the matrix layout tests. None of the changes here seemed to be relevant. I can take a look into this.

[skallweitNV]

I "fixed" the tests by specifying the actual APIs they should run on instead of disabling for metal.

[csyonghe]

Looks good! We can clean up the test discovery logic later to make this more elegant, but we don't have to do that now.