ERP5TypeFunctionalTestCase: make sure activities are not subscribed

Invoking Zuite_waitForActivities when portal_activities is already subscribed and while another thread from this process is already executing activities causes infinite "Got a lock error, retrying...".

This situation happens when developer makes a mistake (runs the tests without unsubscribing activities), so be nice to the developer so that they don't have to restart Zope.