Validate `dotnet test --arch ...` behavior on solution

[Youssef1313]

Generally, RuntimeIdentifier is disallowed on solution level. See #863 and #14281.

The task of this issue:

1. Find the scenario that used to have "double build" when RuntimeIdentifier was allowed (is it simply a console app referencing a library in solution?)
2. Today, dotnet test with VSTest allows runtime identifier. Is it a bug? Does it cause the "double build" issue? Or does, somehow, the bug doesn't happen in this case?
3. With dotnet test with MTP, we should have a test validating this behavior. The test should assert either that the scenario is allowed and no double builds occur, or the scenario is disallowed because a double build will happen otherwise.

[Youssef1313]

FYI @rainersigwald @dsplaisted @mariam-abdulla @nohwnd