Only progress integration tests when there are no pending actions

**Is your feature request related to a problem? Please describe.**
Our integration tests have no concept of pending actions, meaning we often need to run an assertion after each keypress to ensure that the action associated with the keypress has been fully processed before we press the next key.

This causes issues where we can get false positives in tests because we press a key and run an assertion and the assertion holds true _before_ the key was pressed but not after.

We need to introduce a concept of pending actions where lazygit can tell our integration test that there are no pending actions, and therefore the integration test can continue.

This has the additional benefit of making our tests faster because we won't need to retry assertions.

I've made a start on this and I'll raise a PR when it's ready