[SPIRV] Add a means of turning off/on tests for specific spirv drivers

[farzonl]

In the testing of KosmicKrisp Many tests for MoltenVK were XFAIL resulting in unexpected passes. This change makes those tests XFAIL for just MoltenVK.

Results

OFFLOADTEST_GPU_NAME=KosmicKrisp ninja -C ../builds/llvm_relwithdebinfo_offload check-hlsl-clang-vk

Total Discovered Tests: 329
  Unsupported      :  72 (21.88%)
  Passed           : 186 (56.54%)
  Expectedly Failed:  71 (21.58%)

OFFLOADTEST_GPU_NAME=KosmicKrisp ninja -C ../builds/llvm_relwithdebinfo_offload check-hlsl-vk 

Total Discovered Tests: 329
  Unsupported      :  59 (17.93%)
  Passed           : 251 (76.29%)
  Expectedly Failed:  19 (5.78%)

OFFLOADTEST_GPU_NAME=MoltenVK ninja -C ../builds/llvm_relwithdebinfo_offload check-hlsl-vk

Total Discovered Tests: 329
  Unsupported      : 117 (35.56%)
  Passed           : 189 (57.45%)
  Expectedly Failed:  23 (6.99%)

OFFLOADTEST_GPU_NAME=MoltenVK ninja -C ../builds/llvm_relwithdebinfo_offload check-hlsl-clang-vk

Total Discovered Tests: 329
  Unsupported      : 128 (38.91%)
  Passed           : 134 (40.73%)
  Expectedly Failed:  67 (20.36%)