Unify test/publish for helix/local scenarios

[HaoK]

This work item is tracking the unification of local and helix testing, specifically around introducing the ability to build/publish test assets in such a way that the tests are able to run via a new 'config' like mechanism that points at the appropriate assets in a way that makes it easy to configure (local vs helix runs).

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[HaoK]

Meeting notes:

• Goal is to make it easy to be able to run tests against the output of a "dotnet publish" on a test directory that's been xcopied as is (this will give us helix compatibility for free)
• We will use the existing https://github.com/dotnet/aspnetcore/tree/master/src/Mvc/Mvc.Testing/src package to prototype this.
• Idea is to have this package generate a new json file on disk that replaces the attribute burning for things like content roots
• Need to also ideally encompass the Selenium e2e json files too in some way https://github.com/dotnet/aspnetcore/blob/master/src/Shared/E2ETesting/selenium-config.json
• Need to define the appropriate content layout for referenced projects (side by side/subdirs) etc.
• This will take affect Mvc.Functionals (not currently on helix), Identity.Functionals (already on helix, but via msbuild publish copying), Components E2E (not currently on helix)

@dotnet/aspnet-blazor-eng please review my notes