Validation of order of initial/start/stop/final cycle points

[wxtim]

Closes #6866

Follow-up to #4302

Apparently there was no systematic validation of whether users were supplying sensible values for these, except in a small number of cases.

In this PR, I write a thorough set of cases and ensure that they all pass.

Currently this targets 8.4.x, but I understand that given the refactoring it might be sensible enough to target 8.6.0.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

Broken the one test which did test any case

[oliver-sanders]

(assigning commenters as reviewers)

[wxtim]

Draft because it's broken loads of tests

[wxtim]

@MetRonnie @hjoliver Poke.

[wxtim]

Bump

[MetRonnie]

Unresolved question Flag this as a question for the next Cylc project meeting. : #6874 (comment)

Also, base branch is 8.6.x but milestone is 8.7.0 - one of these needs to change