Enable open-source signing product and test binaries.

Update our dependency on dotnet/buildtools to Microsoft.DotNet
.BuildTools 1.0.12-prerelease, which includes support for open-source
signing.  Update InternalsVisibleTo to test assemblies to reference
by strong name.
Merge branch 'enable-signing'
Merge branch 'master' of https://github.com/dotnet/corefx