Remove instantiated (child process) objects from TestFailure

Resolves #1351

Let's keep communication centralized over there.
To clarify:  I'm aware that PHPUnit's current code contains almost no developer documentation at this point.  I don't know whether that has been an informed decision or whether it's just caused by laziness.

However, in case of deeply internal PHP gems like this, it's guaranteed that every contributor who studies the code will have no idea why PHPUnit is doing what it's doing (unless she's familiar with the problem space already, which is very unlikely).  Having to grep commit logs first, just because some code happens to resolve a complex problem space, presents a barrier to contribution.  That's to be avoided.

Therefore, it's best to make it easy for new contributors to understand (at minimum) architecture-level considerations and pitfalls.  Doing so is a qualified and verified approach to increase contributions.

_(In fact, being deeply involved in one of the largest and most thriving FOSS communities, I won't hesitate to say that this is the key to sustainable success, specifically with regard to the quality and amount of user contributions.)_