There are a dozen extensions that we ship in the tarball as key MediaWiki extensions. They should always work with every change to MediaWiki; it shouldn't be possible to merge code that breaks them. With https://gerrit.wikimedia.org/r/#/c/436190/ we're now testing seven of these extensions, but not the rest.

This is not trivially possible right now because we run this as a single test, and e.g. MediaWiki's own parser tests are disrupted by the MediaViewer extension's changes to images' HTML output: T69302: MultimediaViewer thumbnailBeforeProduceHTML hook breaks other extensions parser tests.