feat: Refactored acceptance test automation

[stevehipwell]

Resolves #2425

Depends on #2511
Depends on #2501
Depends on #2502
Depends on #2506
Depends on #2507
Depends on #2509

---

Before the change?

• The acceptance tests are complicated and don't all pass
• The acceptance tests can't be run on a PR.

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
  • Enterprise tests and external user tests have been skipped

Outstanding Tasks

• Add acctest repo label
• Configure non-anonymous test environment
  • Test user 1
    • Token
  • Free org (owned by test user 1)
    • Repo
    • Template repo
    • App installation
  • Test user 2
    • Member of free org
  • External test user 1
    • Token
  • External test user 2

Optional Tasks

• Configure non-anonymous test environment
  • Team org (owned by test user 1)
    • Membership for test user 2
    • Repo
    • Template repo
    • App installation
  • Enterprise (owned by test user 1)
    • Membership for test user 2
    • Enterprise org
      • Membership for test user 2
      • Repo
      • Template repo
      • App installation
• Add dotcom repo environment?
• GHES?
  • Add ghes repo environment

Pull request checklist

• 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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[stevehipwell]

@kfcampbell I've got the anonymous tests running correctly after refactoring the acceptance tests and defining all of the inputs and when they should run. I'm now at the point where I need access to test users and organisations, I've updated the description above to cover what's needed.

[stevehipwell]

@kfcampbell @nickfloyd I've got all of the acceptance tests running locally that don't need paid functionality or additional users, which is most of them. We're now just waiting on some maintainer input.

[stevehipwell]

This PR now depends on #2511 to have been merged first so that the tests can be run from this PR. This is still waiting on some repo and external setup tasks, but I've reduced the required list to be "good enough".