UID, GID, and privilege control in tests

It's currently difficult-to-impossible to write tests that involve switching UID/GID and deal with changing privileges. This has come up in #6115, and is the reason behind much of the code in the [setgid tests](https://github.com/google/gvisor/blob/fcad6f91a3f292b6b76be10f03baf05ee5245d3d/test/syscalls/linux/setgid.cc). Test coverage would be better if we could control the more system state (test process UID/GID, other UID/GIDs present) of tests before they launch.