test: e2e scenarios

[jthegedus]

Is your feature request related to a problem? Please describe.

It would be good to get some e2e tests that ensured the adapter output was in a working state once deployed to an environment.

Running svelte-kit build followed by the Firebase emulator should be possible in CI with some curls and assertions on the HTML responses.

Describe the solution you'd like

A self-contained CI implementation, preferrably using the Firebase emulator.

Describe alternatives you've considered

Deploying to an actual Firebase project.

Additional context

[jthegedus]

Related: sveltejs/kit#639

A clone of the example app built for testing the official adapters would be ideal.

[jthegedus]

#52 introduced integration tests with 2 example apps, though we need more integration tests and e2e tests.

Since Cloud Run deployments cannot be tested using the Firebase Emulator, we will need to actually deploy to production projects and check curl responses.

[jthegedus]

#123 added e2e tests with the Firebase Emulator