test(helpers): pass client instead of pool; fix typing

[bchrobot]

Description

This fixes passing the client to test helpers instead of the pool. It also adds typings to the pools cache to allow TS to catch future mismatches.

Performance impact

Presumably none.

Security impact

Presumably none.

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.

[benjie]

Thanks!