Testing objects with unit tests

Since web tests now take considerable time to execute, due to the improved scheme, testing detailled functionality specifications with unit tests seems like a good idea. Especially objects are, I think, pretty fit for being tested in such a way, as they practically automatically represent such units (at least in some cases).
However, since object creation now (with the registry) needs database access, I can't get unit tests using objects to work. If this is a known limitation, it should be mentioned in the documentation (if it is, I didn't find it). If there is a way to do it, I'd appreciate some help here (and that, too, should probably be mentioned in the documentation).

The error I'm getting is this:
An AJAX HTTP error occurred. HTTP Result Code: 200 Debugging information follows. Path: /drupal/drupal-7.x-dev/batch?id=108&op=do StatusText: OK ResponseText: Fatal error: Class 'SearchApiIndex' not found in sites/all/modules/search_api/search_api.test on line 414

The only work-around I could right now imagine is to include all needed files manually, but with all the dependencies this would be rather tedious, I think.
What would be the best approach here?