This PR adds a comprehensive test that ensures that no item-closing
action will panic when no items are present. A test already existed
(`test_remove_active_empty `) that ensured `CloseActiveItem` didn't
panic, but the new test covers:
- `CloseActiveItem`
- `CloseInactiveItems`
- `CloseAllItems`
- `CloseCleanItems`
- `CloseItemsToTheRight`
- `CloseItemsToTheLeft`
I plan to do a bit more clean up in `pane.rs` and this feels like a good
thing to add before that.
Release Notes:
- N/A