Update DXC ExecutionTests to not need a HlslDataDir

[damyanp]

It is annoying to have to specify /p:HlslDataDir on the command line, and if you're missing it, the error message produced is non-obvious.

[jenatali]

We've learned that this is a regression specifically in how the execution tests are integrated into other execution environments which stems from #7875, essentially causing the XML file loading to no longer inherit a define that it previously did.