Add test coverage for interruptible with child pipelines

What does this MR do and why?

Adds more test coverage for interruptible child pipelines. https://docs.gitlab.com/ee/ci/yaml/#interruptible

This is added because interruptible breaks for the solution proposed in #273378 (closed).

There are a few gaps in coverage. The first step is to ensure all test cases are covered that are documented so we don't end up with accidental breakage.

Related to #273378 (closed), !82149 (closed)

How to set up and validate locally

1. Run the spec

   bin/rspec spec/lib/gitlab/ci/pipeline/chain/cancel_pending_pipelines_spec.rb

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.