Consider renaming CoreFx.Private.TestUtilities

[ViktorHofer]

We are called .NET Libraries Team 🤙

cc @danmosemsft @stephentoub

[danmoseley]

"Runtime" or "Libraries" might be fine.

[ViktorHofer]

The old name was chosen because we had to package that library and uploaded it to our private feeds. We aren't doing that anymore which means we probably want something simpler like just TestUtilities.dll?

[danmoseley]

SGTM