Askrene: Reservation leak under load

[ScuttoZ]

Important

26.04 FREEZE March 11th: Non-bugfix PRs not ready by this date will wait for 26.06.

RC1 is scheduled on March 23rd

The final release is scheduled for April 15th.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade

Summary

• Adds a test for reservation leaks under load in askrene
• Covers the scenario described in PR Reservation leak test and mitigation #8681

[cdecker]

Rebased on top of master to see if it passes.

[Lagrang3]

The first 2 commits are from another PR they can be removed.
Also I think the last commit was introduced by mistake, please don't remove my simple regression test.

[Lagrang3]

@ScuttoZ, tests were failing with the concurrent payments' futures timing out.
I've increased the timeout 10 fold.

[Lagrang3]

Thanks @ScuttoZ.
Keep bringing those tests!