Improve snapshot testing for Optimization Detective

[westonruter]

Writing snapshot tests for Optimization Detective has been tedious since the buffer HTML and expected HTML have been embedded in PHP files. This has made adding new tests tedious by needing to ensure the HTML is properly escaped. The HTML in the PHP files was also indented additional tabs for readability, and when attempting to paste updates to the expected output into the PHP, this would often cause problems with auto-formatting.

Previously test cases were contained in single PHP files which returned an array with set_up, buffer, and expected keys. With this PR, now tests are in separate directories with set-up.php, buffer.html, and expected.html files. Whenever the buffer.html and expected.html files do not match during tests, then the test will emit an actual.html (which is ignored from git) which the developer can then simply rename to expected.html to update the test. This is also the case when creating a new test, where you just add a set-up.php and buffer.html and the test will then issue an "test incomplete" warning while writing out the actual.html for the developer to then examine and rename to expected.html if it is all as expected.

This PR also removes duplicated test set up logic between Optimization Detective, Image Prioritizer, and Embed Optimizer.

This PR is massive, but the changes in plugins/**/test-cases/* are just splitting up the PHP files into the set-up.php, buffer.html, and expected.html files. So these do not need to be reviewed. The only exception may be the set_up logic in video-with-large-poster-and-desktop-url-metrics-missing and video-with-large-poster-and-desktop-url-metrics-collected which change/improve how the image URLs are inserted into the buffer using placeholder replacements.

Note that phpunit-snapshot-assertions cannot be used because it requires PHP 8.1.

Fixes #1418

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.43%. Comparing base (4c1aa71) to head (b059cbf).
Report is 262 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #1791   +/-   ##
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  Coverage   57.43%   57.43%           
=======================================
  Files          84       84           
  Lines        6508     6508           
=======================================
  Hits         3738     3738           
  Misses       2770     2770           

Flag	Coverage Δ
multisite	57.43% <ø> (ø)
single	34.55% <ø> (ø)

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[swissspidy]

Nice evolution on how tests for Optimization Detective are written! This will definitely speed up development going forward.