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.
|MultimediaViewer||$wgUseInstantCommons = true;||Probably has a test for instant commons but we do not want it enabled by default in MediaWiki|
|Done||Popups||$wgUsejQueryThree = false;||That setting has been removed from MediaWiki|
|ORES||various||Points to testwiki which might not exist, define models|
|VisualEditor||$wgVisualEditorShowBetaWelcome = false;||Disable the welcome menu which breaks MediaWiki core Selenium tests|
|MobileFrontend||various||Set a beta mode, wikibase settings, hook for an es interwiki|
|FileImporter||$wgEnableUploads = true;||The user must be allowed to upload for this extension to work.|
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