tunables: Fail tests correctly when setgid does not work

The child process of the tst-env-setuid process was failing correctly
with EXIT_UNSUPPORTED but the parent did not carry that status forward
and failed instead.  This patch fixes this so that tests on nosuid
/tmp fails gracefully with UNSUPPORTED.  Tested by making my tmpfs
nosuid.

	* elf/tst-env-setuid.c (do_execve): Return EXIT_UNSUPPORTED in
	parent if child exited in that manner.  Print WEXITSTATUS
	instead of the raw status.
	(do_test_prep): Rename to do_test.
	(do_test): Return the result of run_executable_sgid.
	(TEST_FUNCTION_ARGV): Adjust.