fix(tests): don't leave emulator process running

[mmilata]

When an exception is raised in the wrong place, pytest exits but leaves emulator process running in the background. This can result in confusing behavior of further pytest invocations which will connect to this old emulator. The fix attempts to keep track of running emulators in a global variable and then kill them in atexit() hook.

Included are two more quality of life improvements.

[github-actions]

core UI changes	device test	click test	persistence test
T2T1 Model T	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3B1 Safe 3	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3T1 Safe 5	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
All	main(screens)

[romanz]

LGTM, thanks!