Show dialog when running .NET 6 GUI single-file on 5.0 hostfxr

* Rename test directory to tests for Installer and update .sln file
* Update eng/Subsets.props file

This matches Arcade convention.


Commit migrated from https://github.com/dotnet/core-setup/commit/8d28847eac575c9b4166057f5d54cee2699e3df2

Commit migrated from https://github.com/dotnet/core-setup/commit/18780678da576d8c629066f50af30159a8859b2f

* Framework resolution tests

Backfill detailed tests for framework resolution behavior, specifically
around roll-forward.

Introduces some new test helpers
* hostpolicy mock, to avoid actually loading and running coreclr just to
validate selected frameworks
* Builder-like API for `.runtimeconfig.js`
* Builder-like API for shared frameworks
* Some helper classes to limit file copy while keeping the tests fully
isolated


Commit migrated from https://github.com/dotnet/core-setup/commit/e42109d82608c0c0a6123f9b70c21ab632b08d07

Introduce the component resolve entry point.

- Refactors argument init to use the same code for app and components
- Remove usage of the global init structure from args init - makes it much clearer what is used where.
- Other small refactorings
- Adds basic doc with notes and open questions - should eventually become the real design doc for this feature.

Commit migrated from https://github.com/dotnet/core-setup/commit/2fbde0a1c6787b1eee2bd57589dae9b0e0f6a2ea

Commit migrated from https://github.com/dotnet/core-setup/commit/8fe49b4211abaa649784966a4f6b55a38a237d9e

Commit migrated from https://github.com/dotnet/core-setup/commit/ef0dbddcf672f38ecc830ebbf51febf4c71c1879

Commit migrated from https://github.com/dotnet/core-setup/commit/6eba1b53a03c642e0170e7b9b7df2e7722efafe3