Enable open-source signing product and test binaries.

Update our dependency on dotnet/buildtools to Microsoft.DotNet
.BuildTools 1.0.13-prerelease, which includes support for open-source
signing.  Update InternalsVisibleTo to test assemblies to reference
by strong name.

NOTE - the build tools this change depends on is in PR.  You can view it here:
https://github.com/dotnet/buildtools/pull/11

To test these changes locally, pull and build the above pull request as a NuGet package version "1.0.13-prerelease", and consume that NuGet package in this build.  You can do that by:

Producing the NuGet package:
1) pull changes
2) add a tag with the version number to the latest commit:
     `git tag Microsoft.DotNet.BuildTools-1.0.13-prerelease`
3) building the package with "build.cmd"

Consuming the NuGet package:
1) Create a location for local packages on your hard drive
2) Copy the built buildtools package to this location
ie. copy "D:\oss\buildtools\bin\Release\Packages\Microsoft.DotNet.BuildTools.1.0.13-prerelease.nupkg" "D:\Packages\"
3) Update corefx NuGet.config to consume your local package source

    diff --git a/src/nuget/NuGet.Config b/src/nuget/NuGet.Config
    index fb44517..c9aea8d 100644
    --- a/src/nuget/NuGet.Config
    +++ b/src/nuget/NuGet.Config
    @@ -4,6 +4,7 @@
         <add key="enabled" value="True" />
       </packageRestore>
       <packageSources>
    +    <add key="test-local" value="D:\Packages\" /> <!-- DO NOT COMMIT!  IF YOU SEE THIS IN A CR/PR, SAY SOMETHING! -->
         <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
       </packageSources>
       <activePackageSource>

4) Build corefx using "build.cmd"
Thanks!  I made the change and am updating the PR.