tests: Add helper for TIOCSTI exploit

tests: Add helper for TIOCSTI exploit

This helper/exploit injects "id -u -n\n" to the vulnerable calling terminal.

Use id -u -n to get a reproducible output of test cases based on it.

What can happen:

Nothing, no exploit: pty is not accessible, sedsid() disconnected the task from
pty, TIOCSTI failed.

The command is injected to the unprivileged environment pty, and you see e. g.
"nobody": This is acceptable.

The command is injected to the caller (privileged) pty, and you see "root" (or
caller uid name): This is not acceptable and has security implications.

References:

CVE-2016-2779
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2016-2779
http://seclists.org/oss-sec/2016/q1/448
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=815922
https://bugzilla.redhat.com/show_bug.cgi?id=173008
https://bugzilla.suse.com/show_bug.cgi?id=968674
https://bugzilla.suse.com/show_bug.cgi?id=968675

CVE-2016-2781
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2016-2781
http://seclists.org/oss-sec/2016/q1/452

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>
Cc: Federico Bento <up201407890@alunos.dcc.fc.up.pt>