Generate test_case_started/finished messages at the source

[brasmusson]

Description

Generating messages at the source means that the Runner shall generate test_case_started/finished and test_step_started/finished (at least)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds new behaviour)

Please add an entry to the relevant section of CHANGELOG.md as part of this pull request.

Note to other contributors

If your change may impact future contributors, explain it here, and remember to update README.md and CONTRIBUTING.md accordingly.

Checklist:

Your PR is ready for review once the following checklist is
complete. You can also add some checks if you want to.

• Tests have been added for any changes to behaviour of the code
• New and existing tests are passing locally and on CI
• bundle exec rubocop reports no offenses
• CHANGELOG.md has been updated

[luke-hill]

I get the premise of this, but there's a few questions I have regarding how this is used as on first inspection some of these ivars won't change