zope.app.twisted FTP tests should not leave pending calls and threads behind

[Sidnei da Silva]

After updating an internal project to use Twisted 8.2, I had to change the ftp tests to get them to pass. One thing that changed was the '_Janitor' class, which doesn't have a 'do_cleanThreads' method anymore. Instead, I had to replace that by the following code:

+ janitor = util._Janitor(None, None)
+ janitor._cleanPending()
+ janitor._cleanThreads()
+ janitor._cleanReactor()

This code should not need to be called to get the tests to pass. Instead, the tests should cleanup after themselves.