refactor: improve e2e testing

[JosephLenton]

This is still a WIP.

Problem

Dropping support for axum-test-helper in favour of axum-test, and using axum-test to simplify test setup.

Changes

There should be no product or functional changes.

All changes are internal changes to testing.

Does this work well for both Cloud and self-hosted?

Yes.

How did you test this code?

• Ran tests locally
• Ran tests on CI