Work around potential deadlock when stopping threaded test

For example, with test2Clusters:

Traceback (most recent call last):
  File "neo/master/app.py", line 111, in run
    self._run()
  File "neo/master/app.py", line 131, in _run
    self.playPrimaryRole()
  File "neo/master/app.py", line 318, in playPrimaryRole
    self.provideService()
  File "neo/master/app.py", line 269, in provideService
    em.poll(1)
  File "neo/lib/event.py", line 90, in poll
    self._poll(timeout=timeout)
  File "neo/tests/threaded/__init__.py", line 150, in _poll
    Serialized.tic(self._lock)
  File "neo/tests/threaded/__init__.py", line 102, in tic
    cls.release(lock); cls.acquire(lock)
  File "neo/tests/threaded/__init__.py", line 88, in acquire
    elif threading.currentThread() in cls.pending:
TypeError: argument of type 'int' is not iterable

neo/tests/threaded/__init__.py

@@ -82,10 +82,11 @@ class Serialized(object):
         if lock is None:
             lock = cls._global_lock
         lock.acquire()
-        if type(cls.pending) is frozenset: # XXX
+        pending = cls.pending # XXX: getattr once to avoid race conditions
+        if type(pending) is frozenset: # XXX
             if lock is cls._global_lock:
                 cls.pending = 0
-            elif threading.currentThread() in cls.pending:
+            elif threading.currentThread() in pending:
                 sys.exit()
         if cls._pdb:
             cls._pdb = False