chore: Refactor integration tests

[stevehipwell]

This PR replaces #2476.

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Before the change?

• The acceptance tests are complicated and don't all pass

After the change?

• Acceptance tests have been refactored
  • Fewer tests
  • Simpler test structure
  • Test env setup has been made more granular and documented
• Acceptance tests have been tested
  • Tests introduced after v6.4.0 haven't been tested
  • Enterprise tests and external user tests have been skipped

Outstanding

• Re-run all tests locally

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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