Currently, MediaWikiIntegrationTestCase::getExistingTestPage() defaults to the constant title UTPage. This means that tests which don’t properly clear all database tables can have state related to this UTPage page leaking between them, as several tests will all target the same page; such issues can be quite tricky to debug (see T340004 and T341210 for an example where this happened in FlaggedRevs).
To avoid this, I think getExistingTestPage() should choose a random title (it could still start with UTPage as a prefix), like e.g. getNonexistingTestPage() already does ('UTPage-' . rand( 0, 100000 )).