feat: Refactored acceptance test automation

[stevehipwell]

Resolves #2425

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Before the change?

The automated GH Actions integration tests are broken and highly complex.

After the change?

The GH Actions integration tests work and have been simplified to run all relevant acceptance tests for github.com.

The logic to trigger the tests is to either manually run them or to add a label (acctest?) to a PR. Anonymous tests should run as is but there should be dotcom and ghes environments protecting the relevant secret and the environment should require approval.

Outstanding Tasks

• Update contributing docs
• Add acctest repo label
• Add dotcom repo environment
• Add ghes repo environment
• Configure non-anonymous test environment
  • Test user (owns)
    • Token
    • Free org
      • Repo
      • Template repo
      • Secret
      • App installation
    • Team org
      • Repo
      • Template repo
      • Secret
      • App installation
    • Enterprise
      • Enterprise org
        • Repo
        • Template repo
        • Secret
        • App installation
  • Org user
    • Member of free org
    • Member of team org
    • Member of enterprise org
  • External user 1
    • Token
  • External user 2

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.