[tests/engine/singledbtest] Allow testing multiple databases

Igor Poboiko requested to merge poboiko/baloo:dbtest into master

Increase the maximal amount of subdbs for tests to 12 (number currently used in Baloo). This won't hurt, and allows to reduce code duplication in DocumentUrlDBTest, which wants to test two subdbs simultaneously.

Since it's technically no longer a SingleDBTest, the base class is renamed to DBTest.

This is also used in !91

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