Adds very basic property tests for navigation within a single pane, with
these operations:

```rust
enum Operation {
    Open { path: Arc<RelPath>, allow_preview: bool },
    GoBack,
    GoForward,
}
```

It primarily tests that a single backward or forward navigation step
results in the correct item being active. It doesn't currently test that
going back multiple times in a row results in the correct items because
the deduplication logic that Zed uses for this turned out to be hard to
model in an "obviously correct" way, and I aimed to keep it really
simple.

These tests surfaced a navigation bug that this PR also fixes, where
replacing an inactive preview tab would sometimes corrupt the navigation
history.

Before:


https://github.com/user-attachments/assets/52e21002-1319-4b16-93d7-8f84fb8858c0

After:


https://github.com/user-attachments/assets/790b6e3b-4c2b-4f2e-a705-70b1bc034872

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed a bug where replacing a preview tab would sometimes break
navigation.

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>