workspace: Add basic pane property tests and fix navigation bug

[timvermeulen]

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

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:

before.mov

After:

after.mov

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

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

[probably-neb]

nice! this is super cool. If you wouldn't mind fixing the conflict in Cargo.lock, i'd be happy to merge this

[timvermeulen]

nice! this is super cool. If you wouldn't mind fixing the conflict in Cargo.lock, i'd be happy to merge this

Awesome! I've just done so.