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>