When running Selenium tests locally or on CI on a per patchset basis, we went with introducing a way for extension to use different settings. Many projects do not need extra settings at all, we should look at all `tests/selenium/LocalSettings.php` and possibly have sane default settings that would render it useless. That might not be achievable though.
Examples:
| Status| Project | Setting | Description
|--|--|--|--
| | MultimediaViewer | $wgUseInstantCommons = true; | Probably has a test for instant commons but we do not want it enabled by default in MediaWiki
| {icon check} Done | Popups | $wgUsejQueryThree = false; | That setting has been removed from MediaWiki
| | ORES | various | Points to `testwiki` which might not exist, define models
| | VisualEditor | $wgVisualEditorShowBetaWelcome = false; | Disable the welcome menu which breaks MediaWiki core Selenium tests
| | MobileFrontend | various | Set a beta mode, wikibase settings, hook for an `es` interwiki
Related, a discussion @zeljkofilipin and @hashar had a few months ago about PHPUnit tests: https://docs.google.com/drawings/d/1ebVAFqM80Me7HCiLV-JdAkNCh3unLCGsuNqRcgiGtEI/edit?usp=sharing