Moved from T197469#4642964
The "mwgate" build, which runs tests with a core set of extensions installed, only executes the "extensions" suite (--testsuite extensions argument to phpunit) -- it doesn't execute the core tests with extensions installed.
That means that core tests can/will break when certain extensions are installed, for example:
https://gerrit.wikimedia.org/r/c/mediawiki/core/+/463383 (OutputPageTest broke when Translate was installed)
We should (re)add a job which installs all the extensions and then runs core tests.