Make it possible to emulate events in an offscreen canvas

[almarklein]

There is canvas.submit_event() that was added for this purpose, but I'm actually not sure if the offscreen canvas emits its events properly, since it does not have a proper loop.

Let's add some tests to make sure emulating tests is possible. Then downstream code can use event-emulation to test some of their interaction logic.

Ref fastplotlib/fastplotlib#705