Enable running tests against .NET Framework

[danmoseley]

Right now the tests are only built and run against netcoreapp2.1. We want to be able to run our unit tests on .NET Framework as well, as we will support that when we release 1.0

1. The build part is because of here. The tests either need to be built against netstandard2.0 or multitargeted. Building against netstandard2.0 (modifying the line above) fails because the tests depend on the package xunit.runner.visualstudio 2.4.0 which it happens does not have support for netstandard2.0. So probably this line needs to be changed to something like <DefaultTestTargetFramework>net461,netstandard2.0</DefaultTestTargetFramework> (comma separated) so that it produces assets for both. Not sure which version of .NET Framework to use here. Whichever it is, the developer will need reference assemblies installed so perhaps we can pick whatever the default is for those.

2. The running part I did not investigate. Maybe the above is sufficient and it's just another flag to pass to run against .NET Framework.

[danmoseley]

cc @ericstj @Anipik