That test goes together with thread detach, but I am also calling joins
to make sure we're not breaking them. It is unfortunate that this is quite
non-deterministic and we can't really surely test for failure. But on the
flip side, it did help me catch a couple of bugs in my implementation. So
it will eventually explode somewhere if a bug appears.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>