Adapt parentless spawning tests

[hjoliver]

Close #7359
(Follow-up to #7237)

Notes 1: I removed Cylc 7 compat tests rather than adapting them, as they'll be removed soon anyway.

Note 2: In most start(schd) integration tests (which don't start the main loop) I ended up using a new fixture to spawn parentless tasks to the runahead limit rather than adapting the tests to work via direct manipulations of the initial task pool. This is the right thing to do because parentless tasks are logically free to spawn ahead without any other changes occurring (even though the main loop is what does that normally), and we need tests that examine the logical end-state of a given set-up or manipulation. The fixture allows that without running the main loop (which would generally evolve the task pool beyond the aforementioned state via task completion etc.).

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.

[hjoliver]

and we need tests that examine the logical end-state of a given set-up or manipulation.

[w.r.t. parentless spawning to the runahead limit]

Many many tests of course make use of parentless tasks to see the effect of some action on future cycles, since they automatically spawn there without needing upstream tasks to complete.

[MetRonnie]

Drafting while #7237 (comment) is yet to be addressed

[hjoliver]

Can be undrafted once #7400 is merged.