Disable running TPL tests by default.

On .NET Core, the TPL tests are hanging and need investigation.
Disable them by default on the command line build for now.
Fix merge error in MetadataReader test project
Add nuget package reference to myget dotnet-buildtools feed

Add reference to https://www.myget.org/F/dotnet-buildtools/ which
today has Microsoft.DotNet.BuildTools and Microsoft.DotNet.TestHost.