ENH: add kwarg "interactive" to PsychoPyApp onInit

Setting interactive=False would allow avoid dialogs while unit-testing the app
ENH: use conftest.py and pytest_configure to setup a fixture to start the app in non-interactive mode

this is py.test specific solution but should be more robust.  I have removed various hanging around setup*s:
setup_module seems to be not invoked whenever testing only submodule etc

also avoiding use of globals here by assigning to the actual app class ;)
ENH: use setup_class fixture instead of global space pollution to initiate a single Experiment for tests