[SER] Reorganize SER tests

[simoll]

"I think AST tests belong under SemaHLSL. We could probably address this later for these tests, but in the future, we should be putting them there. For -fcgl and full compilation, that should go here, but we also need to add a lit.local.cfg under CodeGenDXIL/hlsl/objects/HitObject to require dxil-1-9. This is not a requirement for SemaHLSL tests."

Originally posted by @tex3d in #7262 (comment)