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"
The reasons are for time, and parity with what we do today.  I'll include this in the detail of the issue to move us to using our proper nuget fx packages.