Add tests for dev mode to the app blueprint

[BlueCutOfficial]

We were having issues with EsBuild resolver when we spotted that the CI of the app blueprint wasn't able to notify us about the EsBuild resolver errors happening when starting the generated app. This is because the tests of the app blueprint always use a Vite build and don't start the Vite dev server.

I think this is an issue, because after using the blueprint, the first thing a developer would do is starting the dev server, and the CI of the app blueprint should let us know it goes well.

[mansona]

So #38 gets us most of the way there, but we should still consider implementing a puppeteer/playwright test to further verify that things are ok.

if the test that I'm linking above passes then we know that the esbuild process isn't throwing any errors, but we don't know if the runtime timing of all the optimized dependencies (e.g. ember-source) is correct so we should spin up a browser and make sure that things are actually working 👍 remember this can't be done with ember test starting a real browser because that relies on ember build which uses the rollup version of vite and not the esbuild version of the code 🫠

[ef4]

This is now well-covered.