I see that the extension has its own phpunit.xml file. I wonder whether we should just get rid of it, given that it's non-standard. Or at least keep it compatible with core's PHPUnit settings, but that sounds challenging...
I don't think there's one. Apparently, it was introduced because it was faster than MW's PHPUnit config. But now in 2019 we have MediaWikiUnitTestCase etc., so let's just do what extensions normally do.
Of note, this extension is pretty challenging because it's claiming to support MW 1.3+... I'm working on it, too.
https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/MediaWikiFarm/+/545263/ seems to mitigate the issue, but, well... The tests are messing up with globals a lot (amongst other things), so something else broke (as expected).