Introduce "non-runnable" runners and change engine to use them

[CharliePoole]

Part of #1475

[CharliePoole]

This was intended as a very targeted change but expanded a bit in the process. We are no longer using MultipleTestProcessRunner and have switched to using the base class, AggregatingTestRunner directly when there are multiple assemblies. That's because we no longer want to start a process for every assembly - or even for junk files - when they can be quickly disposed of by one of the special runners.

For simplicity, I left the code unchanged in AggregatingTestRunner. It still creates a separate thread per runner, whether it's a ProcessRunner or a NonRunnableTestAssemblyRunner.

Some tests may be non-runnable because a driver can't be found for them, so I had to leave a simple non-runnable driver in the agent core code.