[openmp] [test] Set __COMPAT_LAYER=RunAsInvoker when running tests on Windows

Windows heuristics may decide to want to run some tested processes
as elevated (since it may think some of them are installers - executables
with "dispatch" in the name may hit a heuristic looking for "patch").

Set this environment variable to disable this heuristic and just run
the executable with whatever privileges the caller has.

This fixes a couple tests on such versions of Windows where this
heuristic is active.

Differential Revision: https://reviews.llvm.org/D137772