selftests: tmpfs: Add Test-skip if not run as root

Add 'ksft_exit_skip()', if  not run as root, with an appropriate
Warning.

Add 'ksft_print_header()' and 'ksft_set_plan()' to structure test
outputs more effectively.

Test logs:

Before Change:
- Without root
 error: unshare, errno 1

- With root
 No, output

After change:

- Without root
TAP version 13
1..1
ok 2 # SKIP This test needs root to run!
Totals: pass:0 fail:0 xfail:0 xpass:0 skip:1 error:0

- With root
TAP version 13
1..1

Link: https://lore.kernel.org/r/20250105085255.124929-2-cvam0000@gmail.com
Signed-off-by: Shivam Chaudhary <cvam0000@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c

@@ -23,10 +23,22 @@
 #include <sys/mount.h>
 #include <unistd.h>
 
+#include "../kselftest.h"
+
 int main(void)
 {
 	int fd;
 
+	// Setting up kselftest framework
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	// Check if test is run as root
+	if (geteuid()) {
+		ksft_exit_skip("This test needs root to run!\n");
+		return 1;
+	}
+
 	if (unshare(CLONE_NEWNS) == -1) {
 		if (errno == ENOSYS || errno == EPERM) {
 			fprintf(stderr, "error: unshare, errno %d\n", errno);