main
is invoked with the contents of your ini file. A simple way to load your app from an ini is:
from pyramid.paster import get_app
app = get_app('testing.ini#main')
test_app = TestApp(app)
This expects "testing.ini" to be in the current working directory, so you may need to tweak that. If you'd like it to be relative to a spot in your tree you can use:
import os.path
import some_module
here = os.path.dirname(some_module.__file__)
app = get_app(os.path.join(here, 'testing.ini'))