Avoid rebuilding database fixtures for every test run

Avoid rebuilding database fixtures for every test run

This reduces the runtime of database-bound tests by about 40%
(on my system, from 4:55 to 2:47; results from Jenkins are
inconclusive).

The basic idea is to call addCoreDBData() only once, and have
a addDBDataOnce() that is called once per test class, not for
every test method lie addDBData() is. Most tests could be
trivially be changed to implement addDBDataOnce() instead of
addDBData(). The ones for which this did not work immediately
were left out for now. A closer look at the tests that still
implement addDBData() may reveal additional potential for
improvement.

TODO: Once this is merged, try to change addDBData() to
addDBDataOnce() where possible in extensions.

Change-Id: Iec4ed4c8419fb4ad87e6710de808863ede9998b7