Stop leaking a thread in test_race_between_idle_exit_and_job_assignment

It can lead to confusion, e.g.: python-trio#1604

trio/_core/tests/test_thread_cache.py

@@ -2,6 +2,7 @@
 import threading
 from queue import Queue
 import time
+import sys
 
 from .tutil import slow
 from .. import _thread_cache
@@ -118,3 +119,9 @@ def release(self):
     done = threading.Event()
     tc.start_thread_soon(lambda: None, lambda _: done.set())
     done.wait()
+    # Let's kill the thread we started, so it doesn't hang around until the
+    # test suite finishes. Doesn't really do any harm, but it can be confusing
+    # to see it in debug output. This is hacky, and leaves our ThreadCache
+    # object in an inconsistent state... but it doesn't matter, because we're
+    # not going to use it again anyway.
+    tc.start_thread_soon(lambda: None, lambda _: sys.exit())