runtest/containers: fix errors in netns_par_chld_ftp testcase that takes TCONF as TFAIL

In test case 'testcases/kernel/containers/netns/netns_par_chld_ftp.sh', if iproute tools
do not support setting network namespaces, child process will return TCONF, but parent
process took this as TFAIL wrongly.

So in testcases/lib/test.sh, we introduce tst_record_childstatus()'s shell version, please
see doc/test-writing-guidelines.txt for detailed information this function, it helps to
propagate child process result(child exit status) to the parent process correctly.

In this patch, we fix this issue by using this function.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Acked-by: Poornima Nayak <Poornima.Nayak@in.ibm.com>