Thread reaping in test_support is imperfect because, even when a thread
has a disappeared from _active and _limbo, the Thread object might still
be hanging somewhere in memory (leaving its last instants). This problem
manifests itself when trying to fix the transient refleaks in
test_socketserver.

One solution I've found is to add a _count() method to the thread
module, which returns the number of running threads from the point of
view of the C extension. When _count() is decremented, we can be sure
the Python method has finished running and the Thread object is not
hanging around.