Add option to automatically generate diffs of failing graphical unit tests

## Enhancement proposal
Will allow developers to more easily check how graphical unit tests are failing. One step towards allowing our GitHub CI to automatically post these diffs as well.

## Additional comments

The issue names graphical unit tests and a possible GitHub CI integration, but no files, test entry points, output format, or acceptance criteria. First locate the graphical-test and GitHub CI entry points, then determine how a failing test should produce and expose a diff. Done means the option reliably generates a reviewable diff for failing graphical tests and its CI behavior is defined.

Written by the indexing model from the issue text.