Add EndToEnd tests for major endpoints

[MV-GH]

EndToEndTest success is defined as simply having a 2XX returned for a endpoint. We test serialization.

The only way we can reliably do this is by having a local docker instance running of a LemmyServer using TestContainers. The db should be preseeded with some data.

Then add a test for biggest endpoints: posts, user, site

These tests should only be required to executed before the publishing. On demand otherwise.

[dessalines]

You could look at some of our end-to-end tests here, but they're mostly to test business-logic and federated operations. It might be a little overkill to replicate that with all its seed data here.