This commit adds a check-pstl CMake target that will run the tests
we currently have for pstl. Those tests are not using LLVM lit yet,
but switching them over should be a transparent change. With this
change, we can start relying on the check-pstl target for workflows
Note that this commit purposefully does not support the pre-monorepo
layout (with subprojects in projects/), since LLVM is moving towards
the monorepo layout anyway.