lib: tst_run_cmd: add tst_run_cmd() to exec specified program

vfork() + execvp() specified program.

Acked-by: Cyril Hrubis <chrubis@suse.cz>
Reviewed-by: Caspar Zhang <caspar@casparzhang.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>