Template overrides don't work if the global site manager is stacked

[rpatterson]

The zope.component.getGlobalSiteManager may be stacked (using __bases__) such that if nothing is found for the first site manager, lookup continues down the stack. This is valid ZCA usage and is used, for example, in tests that use plone.testing and plone.app.testing. However, z3c.jbot does some custom ZCA lookup using private internals and doesn't reproduce the stacked behavior. This means that under any stacked global ZCA behavior the template overrides don't take effect. For example, this means that overridden tests aren't testable under plone.app.testing.