test(joiner): add synctest to joiner tests

[akrem-chabchoub]

Checklist

• [x] I have read the coding guide.
• [ ] My change requires a documentation update, and I have done it.
• [x] I have added tests to cover my changes.
• [x] I have filled out the description and linked the related issues.

Description

Add synctest in joiner tests.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

https://github.com/ethersphere/bee/issues/5232

Screenshots (if appropriate):

[akrem-chabchoub]

It would really be easier to review and approve if the change would be just about adding the synctest helpers. I'm not sure if all the other changes are a result of some vscode auto-linter/formatter or a refactor extension, in which case I would disable it at least for these sort of changes. Mind you that these are delicate components. Every tiny change to tests and production code have to be thoroughly understood before being approved.

Yes, I completely agree. If you're using VSCode, these extensions can help avoid noisy diffs and make reviews much clearer:

GitHub Pull Requests: https://marketplace.visualstudio.com/items?itemName=GitHub.vscode-pull-request-github

SemanticDiff: https://marketplace.visualstudio.com/items?itemName=semanticdiff.semanticdiff