More event-based hazard QA tests

Addresses https://bugs.launchpad.net/openquake/+bug/1088864.

The goal of this task was to implement QA tests based on the classical hazard QA tests, cases 2-11. However, for many of these tests I could not get the tests to run in a reasonable amount of time (several minutes) even with relaxed variable precision (see `qa_tests/_utils.py`).

I was, however, able to get cases 2 and 4 working.

So at the moment, we do have an EB QA test with a non-trivial GMPE logic tree (see case 12), but we don't have one for a non-trivial source model logic tree. I wanted to implemented case 7 to satisfy this requirement particularly, but the ran with enough stochastic event sets to require more than 1 hour of computation and still the results didn't quite converge, even with relaxed precision.