Add a generic base for backend tests

Thanks, great work so far!
Attached is a re-roll with the regression test for #2616804: An overlong word (more than 50 characters) was encountered while indexing. I also added a few missing comments and did some rewrite of test entity creation so it's more stable if some methods are overridden (and don't add any entities anymore). Could probably still be improved (i.e., not hard-code the entity IDs – should actually be pretty easy now, come to think of it).

However, the major problem is that the base class is still much too DB-centric. A lot of its methods (e.g., checkMultiValuedInfo(), editServerPartial(), editServerMinChars()) don't make any sense for Solr or other backends.
Instead, the class should cover all the base functionality of backends and then have some clearly defined extension points where child classes can just call whatever methods they want. There, the DB backend would test changes to its specific backend options, and things like the internal DB info, while other backends might do entirely different things.

Also, the doc blocks for the regression tests don't comply to the coding standards. In the first line, I'd instead really describe what is tested (e.g., "Tests indexing of long multi-byte words.") and then only reference the issues in the extended description (or even with @see tags and proper URLs – not sure).

They are still backend-specific, even if one of them is also available in one other backend. Please either correct this, or wait until I get the time to do it myself.

Patch is not very easy to review, but I think I it looks great, the testbot agrees here and in solr so let's get this in.

Unbelievably, Joris missed a few comment style issues, but apart from that this indeed looks good. Thanks!
After confirming with Markus, I also renamed regressionTestSecondServer() to checkSecondServer().
Otherwise, looks good. Committed.