Add two spec examples of canceled CI builds

What does this MR do and why?

This MR adds test cases for this;

(from the Slack thread)

The canceled and failed statuses sometimes behave the same, sometimes not...

1. Both statuses are sometimes considered "failed". When a job is failed or canceled with allow_failure: true, we mark their stage/pipeline as "success with warning".

For this example;

build:
  stage: build
  script: sleep 10
  allow_failure: true

test:
  stage: test
  script: exit 0

• If build is canceled, test runs.
• If build is failed, test runs.

2. Sometimes they are different. on_failure only looks for failed for the previous status, not canceled.

For this example;

build:
  stage: build
  script: sleep 10

test:
  stage: test
  script: exit 0
  when: on_failure

• If build is canceled, test is skipped.
• If build is failed, test runs.

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