gtest: tweaked test runner to fix an extra comma, added more tdd-based thread coordinator behavior.

Starting to flesh out the thread state coordinator class that will be used
by Linux/llgs.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218537 91177308-0d34-0410-b5e6-96231b3b80d8