Queue warning

[hjoliver]

These changes partially address #3470 (close after PR port to 7.9.x and master as well)

Requirements 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).

• Appropriate tests are included (unit and/or functional).

• Appropriate change log entry included.

• No documentation update required. (No significant change to users, just fewer warnings)

[hjoliver]

[scheduling]
   [[queues]]
       [[[q1]]]
           members = A, B
       [[[q2]]]
           members = x
   [[dependencies]]
       graph = "x => y"
[runtime]
   [[A]]
   [[B]]
   [[x]]
       inherit = A, B
   [[y]]

On 7.8.x:

cylc-flow-7.8.x$ cylc val foo
WARNING - Queue configuration warnings:
        + q1: ignoring x from B (already assigned to a queue)  <--- SPURIOUS
        + q2: ignoring x (already assigned to a queue)  <--- CORRECT
Valid for cylc-7.8.4-21-gb766b-dirty

On this PR branch:

cylc-flow-7.8.x$ cylc val foo
WARNING - Queue configuration warnings:
        + q2: ignoring x (already assigned to a queue)  <--- CORRECT
Valid for cylc-7.8.4-22-g9db758

[kinow]

Conflicts in CHANGES.md @hjoliver

[hjoliver]

Conflicts in CHANGES.md @hjoliver

(just rebased)

[hjoliver]

I'll replace the new functional test with a unit test, on the master version of this (once this is approved).

[hjoliver]

Note as a follow-on to this change I could change the existing behaviour (as marked <---- CORRECT in the example above) to arguably more standard override instead of warn and ignore. i.e. if a task gets assigned to two queues, the second assignment sticks, not the first. However, I'm unsure if anyone is relying on the current behaviour - what do you think @oliver-sanders ?

Note this is discussed in #2701 along with the possibility of allowing tasks to belong to multiple queues at once, but I don't think we figured out a sensible way of doing that.

[kinow]

Code looks good. Functional tests passed 2 out of 4 jobs. Approving assuming the other two (and flakytests) pass (with or without some kicking 🥋 )

[hjoliver]

(This block was wrongly inside the outermost loop, resulting in repeated warnings in some cases)