XunitTraitsDiscoverers: add reference to System.Threading

This allows the test project to be built with Mono's xbuild.
It's the same change as in https://github.com/dotnet/corefx/pull/288.
Merge pull request #328 from akoeplinger/add-threading-reference

XunitTraitsDiscoverers: add reference to System.Threading
Append with single length strings changed to chars. Updated .tt to match existing code
Merge pull request #325 from bbowyersmyth/chars

StringBuilder append with single length strings changed to chars