Add test for .NET Framework runtime version

[lbussell]

See #1202 for more context.

There should be an image test that validates the .NET Framework Runtime version is exactly what we expect it to be, based on the image's tag. The test should use the registry to check what version is installed.

[lbussell]

[Triage] Before this is implemented, we need to figure out why some images contain the 4.7.2 runtime instead of the expected runtime (e.g. 4.6.2 in the case of ltsc2016 images). See #1202.