Report PRECONDITION_FAILED-ish outcomes for unavailable optional features

[ErichDonGubler]

Quite a few tests in CTS exercise optional features in the WebGPU spec. When these tests' feature dependencies are not satisfied, they currently report PASS (IME; this may be incorrect, viz., some may yield FAIL instead).

According to WPT upstream's "Optional features" section, tests of optional features (i.e., the shader-f16 feature) should use the assert_implements_optional API to emit an PRECONDITION_FAILED outcome. This seems an interesting distinction not only for WPT, but also for the standalone runner's reports.