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

[Tom Stellard via llvm-commits]

tstellar 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?

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