selftests/mm: skip test if application doesn't has root privileges

The test depends on writing to nr_hugepages which isn't possible without
root privileges.  So skip the test in this case.

Link: https://lkml.kernel.org/r/20240101083614.1076768-2-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>