The test doesn't want the child to return an error message after
the parent returns even though this was perfectly reasonable with
our previous design. We were passing the test earlier due to luck
since somehow the child process was terminating early enough due
to jitter.

Still failing multi-oom due to hanging on qemu and bochs.
Still failing syn-read on bochs.

Clean up the thread exit because the child now always exits in one
place instead of three possible places.

Still failing multi-oom.
Also fails syn-read and exec-missing on bochs.

Remove thread from parent list if it wasn't removed already.
All tests work on qemu. Hallelujah! multi-oom still hangs on bochs
and syn-read fails on bochs.

Refactor so we know when an element is removed by checking that
element.

All tests pass on qemu. syn-read has an assertion fail in list_next
and multi-oom hangs on bochs.

All tests passing on qemu.
multi-oom hangs on bochs and syn-read fails.