Allow running tests in a separate domain per assembly, using a separate config file for each assembly.

I saw this on the roadmap, and I might be able to contribute a snippet of code 
to save some time.
 
I've set up TestRunner to work this way from builds .1227 on, basically 
creating a multi-domain runner with a root test suite filled with sub tests 
each in a different TestDomain, then proxying a couple of the events to supress 
multiple test run finished events from firing. It's a faily simple approach. 
The loaded test is visibly/structurally identical to loading a Solution file 
based test.

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