Improve utility of warnings in Iris tests

[trexfeathers]

Story

• As a Iris developer
• I want Warnings that are a problem for Iris to result in CI failure (e.g. a DeprecationWarning)
• So that I can address problems introduced by new commits proactively and immediately (rather than reactively when issues are discovered downstream)

Notes

SciTools/iris#4384

Iris Gallery includes (or has included!) code to convert warnings to failures/errors.

Ideas

• Suppress warnings we know are not a problem, so only those that matter are left. Then set a test run to exit non-0 if any warnings are raised. Would likely need to address quite a few warnings before this could start working!
• A custom routine that causes a failure if it detects warnings of known categories (might only be DeprecationWarning?).

From discussion

• Cost-benefit is difficult. Focussing on DeprecationWarning would be fairly easy, but is it offering much. Covering all warnings may need a lot of leg work to adapt the tests.
• Should start by seeing how many tests fail if warnings are 'promoted', and see how this informs further discussions.

  
  ### Tasks

• [ ] https://github.com/SciTools/iris/issues/5465
• [ ] https://github.com/SciTools/iris/issues/5491
• [ ] https://github.com/SciTools/iris/issues/5492
• [ ] https://github.com/SciTools/iris/pull/5493
• [ ] https://github.com/SciTools/iris/issues/5535
• [ ] https://github.com/SciTools/iris/issues/5534
• [ ] https://github.com/SciTools/iris/issues/5533
• [ ] https://github.com/SciTools/iris/issues/5530
• [ ] https://github.com/SciTools/iris/issues/5531
• [ ] https://github.com/SciTools/iris/issues/5532
• [ ] https://github.com/SciTools/iris/issues/5889

[ESadek-MO]

Note: have converted write up from #5465 into issues and added as subtasks on this issue.