Fix execution time limit polling intervals [cylc 8]

[wxtim]

Response to a bug not otherwise recorded:

Bug description

If the sum of execution polling intervals < execution time limit the remainder should be added to the first, but only the first execution time limit polling interval. Currently the remainder is always added to the first items in the list of execution time limit polling intervals.

The last item in the list of execution time limit polling intervals is repeated until the final itask timeout. At the moment if we set only one item in execution time limit polling intervals then every polling interval is execution time limit polling intervals[0] + remainder.

Example

execution_time_limit = 60
execution_polling_intervals = [50]
execution time limit polling intervals = [10]

list_of_polling_intervals_is = [50, 20] # polls onwards at intervals of 20s ...
list_of_polling_intervals_should_be = [50, 20, 10]   # polls onwards at intervals of 10s ...

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).
• CHANGES.md 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]

The last member of the list is repeated.

if len(time_limit_delays) == 1 then all subsequent delays are not the set execution time limit polling intervals, but that value + the remainder of the execution polling intervals under the time limit.

[wxtim]

This is the bug fix - the rest is refactoring for easier testing.

- time_limit_polling_intervals[0] += time_limit - sum(delays)

[MetRonnie]

Kicking tests

[MetRonnie]

I haven't manually tested but I'm happy that the tests are comprehensive enough