selftests/kexec: define "require_root_privileges"

Many tests require root privileges.  Define a common function.

Suggested-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>