Move shared unit test code to a shared assembly #8364
Description
Inspired by this though #8321 (comment) (by @rainersigwald)
Background
Currently we are sharing functionality in unit test via means of sharing code files - this is less friendly to work with and in case of unittests doesn't bring any actual benefit.
Suggestion
Move all the code from /src/Shared/UnitTests to /src/UnitTests.Shared and replace all inlines of the moved code files with Microsoft.Build.UnitTests.Shared.csproj reference.
In case of hard to solve issues (not sure what those would be though) reverse the action for blocking files and repeat.