Diff viewer for test outputs — compare results between executions

[ThomasMalletCodra]

Description

Add a diff viewer that compares the output of a test between two executions (e.g., current vs. previous, or between two selected campaigns). This helps developers quickly identify what changed when a previously passing test starts failing — especially useful for regression debugging.

Current behavior

• Only the latest test output is displayed.
• No way to compare outputs between runs.
• Debugging regressions requires manually saving and diffing outputs externally.

Expected behavior

• A "Compare" action is available for any test that has been run at least twice.
• Side-by-side or inline diff view highlighting added/removed/changed lines.
• Color-coded diff: green for additions, red for deletions, yellow for changes.
• Option to compare against:
  • Previous execution of the same test
  • A specific campaign (if campaign management is available)
  • A saved "baseline" output
• Diff summary: "42 lines changed, 3 added, 5 removed".

Implementation ideas

1. Store the last N outputs per test (configurable, default: 5) in .moduletester/history/.
2. Use Python's difflib for generating unified or side-by-side diffs.
3. Render diff in a QTextEdit or QWebEngineView with syntax highlighting.
4. Add "Compare with previous" context menu action on test items.
5. Optional: integrate with campaign management (Campaign management/Non regression — execution history and release comparison #9 ) for cross-campaign comparison.

Dependencies

• Enhanced by Campaign management/Non regression — execution history and release comparison #9 (Campaign management) but can work standalone with local history.

Acceptance criteria

• Last N test outputs are stored per test
• Side-by-side or inline diff view is available
• Diff highlights additions, deletions, and changes
• "Compare with previous" action accessible from test context menu
• Diff summary shows number of changed lines
• Output history storage is configurable and can be cleared