Fix subclasses of subclasses of PerformanceTest

`delayedInit` can be called more than once, when multiple classes extend
PerformanceTest in a hierarchy. Accumulate their effects, instead of
discarding all but the last one.
Force again delayedInit to be called

This remedies a regression of the previous commit.
Merge pull request #32 from Blaisorblade/topic/accumulate-tests-in-hierarchies

Topic/accumulate tests in hierarchies