[llvm] [llvm] Add support for running tests as root (PR #75285)

[Thomas Preud'homme via llvm-commits]

RoboTux wrote:

> > How about testing for the ACL bypassing behaviour instead of checking for username == root?
> 
> @arichardson How portable would this check be? Are there any scenarios where this check would return a different result than checking username == root?

binary with setuid bit will have getuser() return the user that ran the binary but the binary will behave as root.

https://github.com/llvm/llvm-project/pull/75285