in package testing for v2 addons

[void-mAlex]

opening this issue to discuss potential solutions for in package testing of code held by a v2 addon

one of the problems faced in testing today's v2 addons are any non app rexported code like private components that would require integration tests
another problem is overhead/difficulty in shipping a few components in a v2 addon that suddenly require a monorepo setup
monorepos also make shipping blueprint updates much more complicated as the monorepo can be set up any number of ways and there could even be multiple test apps for any given v2 addon

one approach could be on the fly package generation like bottled-ember tries to do
looking for what other thoughts are on the matter

[NullVoxPopuli]

For "the simplest scenario", I'd like an in-package app created (not bottled) -- maybe via vite plugin or something (with all boilerplate files internalized / virtualized by default), so that non-external APIs things can be tested -- a real app, regardless how it exists, wouldn't be able to import any of that.