Fix the tests of the event_based depending on random number details

See https://bugs.launchpad.net/oq-engine/+bug/1167310

Some redundant tests were removed by pointing out in that the same features are covered in other parts of the code. The code coverage is less strict than before but enough to discover many incorrect refactoring: for instance moving the calculation of the stochastic event set outside the loop of the event_based.core calculator would break the tests since
the number of calls would change.