Improve integration tests and fix some intemittent tests - #3033

Last comment, otherwise looks good to me :)

There's still a test that fails everytime you run with integration tests: sends status events about running app specs (Livebook.Apps.ManagerTest) (test/livebook/apps/manager_test.exs:116).

I tried the approach to send individual messages only for test env and only to a certain topic and it still intemittent due to concurrency.

@aleDsz the issue is that in that test we expect only one permanent app:

however there are leftover Team hubs from other tests and some of them have app deployments, so there are extra statuses in the list. Normally we could relax the assertions to entry in list, but here it is a bit problematic, because there may be multiple messages and we use the pattern match to wait for a specific one.

I figured the best solution is to make sure there are no leftover Team hubs in the first place, so I added on_exit to do that in fefd552 :)