Allow layer teardown
Make tests pass with zope.app.wsgi 4.0.0a3

Require zope.app.wsgi >= 4.0dev (for allowTearDown=True) for tests.
Avoid eggs cluttering the working tree
Use zope.testrunner because we need layers

Using setup.py collects the tests but fails to call layer setup, causing
a strange error (no ftesting.zcml in the wrong location).