Add BUILD_BVT mode for ctest running as a GHA

[walbourn]

A subset of the test suite should be buildable and run as a GitHub action. The challenge is that it needs to leverage WARP and use a 'headless' render target since there's no guarantees of GPU support in a GHA runner.

[walbourn]

The only command-line tests currently that would work in the cloud are headertest and wavtest. The audio requires an output device, so those command-line tests don't really work either.

Proposal:

• Add command-line test 'DDSWICTest' to do an exhaustive validation of DDSTextureLoader, WICTextureLoader, and ScreenGrab

• Add a command-line test 'APITest' that does basically 'invalid arg' and unit-test style tests of the components with a NULL device and not any rendering