tst_run_cmd: added support for stdout and stderr redirection

Added function tst_run_cmd_fds which allows redirection of
stdout/stderr to file descriptors.

Now function tst_run_cmd is a wrapper to tst_run_cmd_fds.

Signed-off-by: Stanislav kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
lib/tst_module.c: modification for updated tst_run_cmd specification

Signed-off-by: Stanislav kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>