Make test_experimental_notice more targeted in its assertions

[BrynCooke]

Relying on a full snapshot of the log is likely to result in failures either due to environmental reasons or other unrelated changes.

Checklist

Complete the checklist (and note appropriate exceptions) before a final PR is raised.

• Changes are compatible[^1]
• Documentation[^2] completed
• Performance impact assessed and acceptable
• Tests added and passing[^3]
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

[^1]. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this.
[^2]. Configuration is an important part of many changes. Where applicable please try to document configuration examples.
[^3]. Tick whichever testing boxes are applicable. If you are adding Manual Tests:
- please document the manual testing (extensively) in the Exceptions.
- please raise a separate issue to automate the test and label it (or ask for it to be labeled) as manual test

[SimonSapin]

This log message involves non-trivial formatting so it’s unfortunate to no longer test what the full message looks like, but I suppose that’s not worth the fragility this test has exhibited so far.

With this change you can also replace .kill() with .graceful_shutdown(). I used kill to avoid output differences between platforms because graceful_shutdown() is not graceful on Windows so shutdown-related logs end up missing.