thread state coordinator: added test for notify after two pending thread stops.

Glad I did - caught a bug where the auto variable was not a reference
to a set and instead was a copy.  I need to review rules on that!


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218558 91177308-0d34-0410-b5e6-96231b3b80d8