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.
Can we set this in a target? NuGet sets Private=true by default and its too error prone to have to change these manually.