Feature Request: Integration testing using packed extensions

Background
I ran now multiple times into the problem that my tests couldn't catch problems which occurred only on the published version. The reason in most cases was that things are working fine when the extension is launched from the "development working directory" while failing when cleanly installed and installed to the "vscode extension directory" due to file system differences.

Especially when bundling dependencies and excluding files from the packed dependency things can easily go wrong and the test executed will not catch such errors.

Requested solution
For those scenarios it would be awesome if the vscode extension test framework and runners would support a mechanism, where the extension is automatically packed into a vsix (like in npx @vscode/vsce package), installed into a clean vscode instance (--install-extension + --extensions-dir), and then tests are executed using against this instance.

I think I'll be able to stitch something together myself in my CI/CD pipeline but ultimately I'd hope to have something as simple as setting an option in the defineConfig() options.

Start by tracing the defineConfig() options and the existing test runner flow. Compare the requested @vscode/vsce package step with VS Code's --install-extension and --extensions-dir options; done means tests can run against the packed extension in a clean instance through configuration rather than custom CI/CD glue.