When CirrusSearch extension is enabled, tests in SearchEngineTests fail, because CirrusSearch removes DB searches by default from alternatives list, and the test uses DB search engine.
The DB search engine is enabled in setUp() method, but the actual data for the test is added in addDBDataOnce() which is run before that. So when addDBDataOnce() adds pages, they do not get indexed by DB search engine because by that time it is not enabled yet.