trial should warn when there are still running threads in the reactor threadPool

[tom.prince]

The following code demonstrates that it doesn't.

from twisted.trial.unittest import TestCase
from twisted.internet import reactor, defer
from time import sleep

class Test(TestCase):
    def test_test(self):
        d = defer.Deferred()
        def thd():
            sleep(0.1)
            reactor.callFromThread(d.callback, None)
            sleep(1)
            print "HERE"
        reactor.callInThread(thd)
        return d

[exarkun]

Duplicate of #962