test(libp2p): update reacher tests to use synctest

[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 reacher test.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

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

Screenshots (if appropriate):

[acud]

Changes seem to be OK but the CI is failing