threads are not GCed on Mono

Noticed this because some tests (test_hashlib) are running slow and sometimes timing out. Here's a simplified example:

import _thread
import functools
import gc
import threading
import time

def threading_setup():
    threading.current_thread() # ironpython: register the current thread if not running on a known thread
    return _thread._count(), threading._dangling.copy()

def threading_cleanup(*original_values):
    _MAX_COUNT = 100
    for count in range(_MAX_COUNT):
        values = _thread._count(), threading._dangling
        if values == original_values:
            return
        time.sleep(0.01)
        gc.collect()

    print("failed to GC the threads")

def reap_threads(func):
    @functools.wraps(func)
    def decorator(*args):
        key = threading_setup()
        try:
            return func(*args)
        finally:
            threading_cleanup(*key)
    return decorator

@reap_threads
def test_threaded_hashing():
    num_threads = 1

    def hash_in_chunks(event):
        event.set()

    events = []
    for threadnum in range(num_threads):
        event = threading.Event()
        events.append(event)
        threading.Thread(target=hash_in_chunks, args=[event]).start()

    for event in events:
        event.wait()

test_threaded_hashing()

Start by running the simplified Python reproduction on Mono and inspect the values returned by _thread._count() and threading._dangling after gc.collect(). Compare the behavior with the same test on another runtime; done means worker threads are reclaimed and the hashing test no longer reports leaked threads or times out.