Instead of cleaning up the mess in igt_exit make sure we don't even
let it out of the container. See also

commit 75487637


Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Feb 26 22:11:10 2016 +0000

    igt/gem_sync: Enforce a timeout of 20s

which added this helper.

To make sure that everyone follows the rules, add an assert.

We're keeping the cleanup code as a failsafe, and because it speeds
up the testcase I'm following up with.

v2: Chris pointed out that my original patch did nothing. Which I
didn't catch because my testcase was also broken. Unfortunately this
means we need to open code part of the waiting.

v3: The 2nd __igt_waitchildren() isn't necessary, __igt_waitchildren
recovers from EINTR already and keeps waiting (Chris Wilson).

v4: Change the timeout signal vs waitchildren logic to be race-free
(Chris).  This changes the exit code for a timeout from
IGT_EXIT_FAILURE to SIGKILL + 128.

v5: Clarify the docs (Chris).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>