Summarized from https://doc.wikimedia.org/cover-extensions/SimilarEditors/, this describes the basic tests needed to pass the programmatic coverage % checks. On a case by case basis, additional tests might be needed to cover different states/errors/etc. I've noted a few here but it might not be exhaustive.

• Editor.php
• mislabeled coverage doc fixed here: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/SimilarEditors/+/801455

• MockSimilarEditorsClient.php
• add file and tests

• ResultsFormatter.php
• add test for formatRowProperty in ResultsFormatterTest.php
• add test case for when no results returned (from https://gerrit.wikimedia.org/r/c/mediawiki/extensions/SimilarEditors/+/790772)

• ServiceWiring.php
• based on https://doc.wikimedia.org/cover-extensions/GlobalWatchlist/ServiceWiring.php.html, can ignore using @codeCoverageIgnoreStart and @codeCoverageIgnoreEnd

• SimilarEditorsClient.php
• add tests for getEditor and getSimilarEditors to SimilarEditorsClientTest.php

• SpecialSimilarEditors.php
• add tests for __construct, execute, onSubmit, onSuccess to SpecialSimilarEditorsTest.php

• TimeOverlap.php
• needs file and tests