Backport V5 test system to V4

[benjie]

Description

V5's test system is much more pleasant than V4's; but I'd like to bring the V4 tests across to V5 because they test a lot of edge cases. Thus, I'm backporting the testing system from V5 to V4 so that we can later copy the documents and results back to V5 and check for any regressions.

Performance impact

None, only affects tests.

Security impact

None, only affects tests.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.