The REST: Emit swagger spec patch added some validation to the API tests. While doing so, it broke (T352742) CI for a couple of repositories, namely GrowthExperiments, Wikibase and Flow. As far as I understand it, the concept of gated extensions is used to avoid extension CI breaking because of changes in eg. Core.

Since both GrowthExperiments and Wikibase are gated extensions, I'd expect the failure introduced by https://gerrit.wikimedia.org/r/c/mediawiki/core/+/860691 to cause a CI failure right on https://gerrit.wikimedia.org/r/c/mediawiki/core/+/860691. However, that did not happen. The CI failure can be only triggered by uploading an empty DNM CI testing patch, such as https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/980472, and verify the CI passes that way.

Using DNM patches has a couple of downsides: it is expensive in terms of people time, and it also clutters the list of patches. More importantly, I think apitests for gated extensions should run as part of the gate-and-submit tests, so that failures similar to T352742 can be caught by CI (instead of developers trying to figure out why their change does not pass).

Filling this in ci-test-error following an advice given by @hashar.