runltp, pan/ltp-pan.c: record test cases(not fully tested) into file

Since commit 6f6878f4e1406d79cae53564777c5e1245d2a124, we start to
differentiate TCONF from TFAIL, but in pan/ltp-pan.c, we still write
test cases that return TCONF into failcmdfile, this is wrong, fix this.

Meanwhile we add a '-T' option for runltp, to let user record test cases
that return TCONF, namely, by this file, users can know what cases are not
fully tested.

Reported-by: Shuang Qiu <shuang.qiu@oracle.com>
Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>