WikibaseCirrusSearch has a UseCirrus config variable that currently defaults to false, disabling its functionality. This is a confusing default, and can create hassles for developers trying to set it up in a testing environment. It should default to true.
The trouble is that changing the default to true breaks several Selenium tests for the Wikibase and WikibaseLexeme extensions, as shown in the failing test jobs on https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/WikibaseCirrusSearch/+/507597/. Adding a $wgWBCSUseCirrus = false; directive to tests/selenium/LocalSettings.php in WikibaseCirrusSearch does not resolve this problem, and based on my local testing, doing the same directly in Wikibase does not help, either.
Ultimately, we shouldn't be trying to work around the problem; the tests and CI environment should be able to accommodate WikibaseCirrusSearch.