test_event.py: fix test for editing an event
test_event.py: fix test for editing an event

Now there are two separate forms in event-edit page, and we had to
change tests appropriately.

vahtras/amy,swcarpentry/amy,wking/swc-amy,swcarpentry/amy,vahtras/amy,wking/swc-amy,shapiromatron/amy,wking/swc-amy,pbanaszkiewicz/amy,shapiromatron/amy,pbanaszkiewicz/amy,wking/swc-amy,shapiromatron/amy,vahtras/amy,pbanaszkiewicz/amy,swcarpentry/amy