The SummaryFormatterTest service wiring test currently fails when run as a pure unit test because OutputFormatSnakFormatterFactory::getSnakFormatter() uses wfMessage(). We should fix that.
In general, wfMessage() is tricky to replace in service wiring because it depends on the user language, which we don’t want to use in services (compare T279415: Remove calls to deprecated WikibaseRepo::getUserLanguage). However, in this case it actually wants a message in a specific language, so I think we can create a new service (e.g. MessageInLanguageProvider), similar to MessageLocalizer but agnostic of the user language, and use that.