Summary

This PR is specifically to speed up analysis of existing image checker tests. It will likely replace #685 and #695 completely, but ultimately #520/#698 will be needed to 1) automate the pre-test runs of zppy itself and 2) test many more cfg files than the existing ones tested by the image checker.

Objectives:

• Add the ability to test UNIQUE_IDs other than the one on the current branch.
• Consolidate test info and create a markdown results table automatically, as in Automate organization of test results #695, rather than needing to parse for information manually.
• Automatically print lists of missing/mismatched files, as in Automate testing of specific cases #698
• [cancelled for this PR] Automatically create image diff grid, as in Script to generate image diff grid #685.

Issue resolution:

• On the path to resolving Further automate tests #516.

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version [Testing only]
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.