Run tests on .NET Core runtime and framework

This change uses the Microsoft.DotNet.TestHost package to run test
assemblies against a test version of .NET Core, rather than against
the locally installed version of the .NET Framework.  It also adds
the build property "SkipTestAssemblies".  SkipTestAssemblies can be
set to a semicolon delimited list of test assemblies to suppress.
For example:
build.cmd /p:SkipTestAssemblies="System.Linq.Parallel.Tests;System.Col
lections.Immutable.Tests"
It's duplicated in the same way that the path to the package is duplicated in the corefx repository between the packages.config dependencies and csproj reference HintPaths right now.