Test on coreclr

Update our build tools dependency to 1.0.16.

Take in @nguerrera 's change at https://github.com/dotnet/corefx/issues/258
Take in @krwq 's change at https://github.com/dotnet/corefx/pull/237

Update all of our tests to use xunit 2.0, and to be portable libraries.

Temporarily disable tests that fail in CoreCLR against issues.
Why not set the skipped test assemblies in the build.proj file instead of listing them at the commandline?