Remove active sleeps in tests

[jorgemanrubia]

In general, active delays in tests are a bad practice. They slow the suite down, and they are unreliable.

Capybara comes with automatic waiting for the built-in helpers, we should always try to use those when possible. E.g: if something changes in the screen, assert with a selector the expected change, so that Capybara waits until it happens.

I added a custom wait_until helper method for custom conditions that we can't cover with capybara.

I left a couple of sleeps after pressing keys, since I can't think of a way to avoid those.

[jorgemanrubia]

cc @samuelpecher @zoltanhosszu

[zoltanhosszu]

Ah this is awesome!

[samuelpecher]

Not sure if it's this or #479. If I rebuild the assets on main they are different and I get this consistent failure:

Failure:
ActionTextLoadTest#test_Clearing_value_of_editor_doesn't_add_trailing_markup [test/system/form_test.rb:70]:
Expected: ""
  Actual: "<p><br></p>"


bin/rails test test/system/form_test.rb:65