Add VkPhysicalDevicePortabilitySubsetFeaturesKHR:: shaderGraphicsFuzzConformance capability

Some CTS tests fail on shader form/syntax variations derived by the GraphicsFuzz testing framework.

This is most likely fixable with effort within the implementations.

In the meantime, these use cases can be disabled in CTS and reported as Not Supported, by adding a `VkPhysicalDevicePortabilitySubsetFeaturesKHR:: shaderGraphicsFuzzConformance` capability.

This affects 6 CTS tests:

```
dEQP-VK.graphicsfuzz.always-discarding-function
dEQP-VK.graphicsfuzz.cov-apfloat-undefined-matrix-mul
dEQP-VK.graphicsfuzz.cov-bitfieldreverse-loop-limit-underflow
dEQP-VK.graphicsfuzz.cov-instruction-simplify-mod-acos-undefined
dEQP-VK.graphicsfuzz.cov-instruction-simplify-mod-sqrt-undefined
dEQP-VK.graphicsfuzz.spv-copy-object
Fail (Fail)
```

Fixed:
~~`dEQP-VK.graphicsfuzz.stable-binarysearch-tree-false-if-discard-loop`~~

Start by locating the CTS handling for the six named dEQP-VK.graphicsfuzz tests and the portability-subset capability definitions. Determine where shaderGraphicsFuzzConformance belongs, then verify that the affected tests are reported as Not Supported while the already fixed stable-binarysearch-tree test remains unaffected.

Written by the indexing model from the issue text.