See https://gerrit.wikimedia.org/r/c/mediawiki/extensions/DiscussionTools/+/1276079
Apparently we don't run the DiscussionTools test suite with a dependency on MobileFrontend. Instead, they only run when another extension's tests depend on both DiscussionTools and MobileFrontend, such as CheckUser. This confused James and me while we were trying to resolve the CI issues caused by T422031.