setpriority/setpriority01.c: cleanup

* Delete some useless commtents.
* Move the test body from main() to setpriority_verify().
* Expand tests([-20,19]).
* Add tst_require_root for lowering priorities testes.
* Some cleanup.

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>