library: add tst_system for wrapper system(3) without SIGCHLD

The system(3) will raise SIGCHLD signal to parent process, and most test
cases will call tst_sig to poison all signals including the SIGCHLD. So
add system(3) wrapper function to temporarily disable the poisoned
handler for SIGCHLD, which will make the test cases happy. Replace
system(3) with tst_system() to ignore SIGCHLG signal handler poisoned
earlier.

Signed-off-by: George Wang <xuw@redhat.com>
Tested-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Acked-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>