Deduplicate event handler and executor unit tests

[dmjb]

This follows on from the previous PR: simplify the event handler tests to the minimum. This means that the event handling tests (which have asynchronous behaviour and may time out) will not fail when changes are made to the executor logic.

Summary

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Fixes #(related issue)

Change Type

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

Dismissing based on the comment from Puerco. Missed it!

[coveralls]

coverage: 52.436% (-0.003%) from 52.439%
when pulling acb7c3f on refactor-handler-unit-test
into 674252b on main.

[puerco]

Thanks!

[coveralls]

coverage: 52.434% (-0.005%) from 52.439%
when pulling 316acca on refactor-handler-unit-test
into 674252b on main.