- remove separate step to restore Interop projects
- make Interop test asset projects more "normal"
    - use usual mechanism to target latest ASP.NET Core bits
    - remove use of `$(BuildInteropProjects)`
- expand FunctionalTestWithAssets.targets to help w/ local testing
    - still delayed and done only when necessary
    - handle non-Helix testing e.g. handle `dotnet test` too
    - build test assets into location `InteropTests` expects on local machine
- add FunctionalTestWithAssets.props, providing relevant defaults
- expand `FunctionalTestWithAssets` use to Identity.FunctionalTests

nits:
- rename `@(FunctionalTestAssetProjectReference)` to `@(TestAssetProjectReference)`
- cleanup leftover reference to ProjectTemplates/testassets/ in Build.props
    - should have been removed in 54ff3797
- remove `/p:RunTemplateTests=true`; no need to specify the default
- clean up other projects that use `$(MvcTestingTargets)`
    - none need special handling for publish (don't need pages, views, or wwwroot files)