selftests/nolibc: skip the chroot_root and link_dir tests when not privileged

These two tests always fail when the program is started natively as an
unprivileged user, and require the user to carefully check the output
of "make run-user" and ignore them.

Let's add an euid check and condition these two tests to euid==0. Now
the test case stops needlessly reporting failures. E.g.:

  $ make -C tools/testing/selftests/nolibc run-user
  ...
    CC      nolibc-test
  123 test(s) passed.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>