[Doc] Delete duplicate documents [eventmesh-unit-test-rule.md]

[Alonexc]

Search before asking

• [X] I had searched in the issues and found no similar issues.

Documentation Related

Located at: https//github.com/apache/incubator-eventmesh/blob/master/docs/zh/desing-document/eventmesh-unit-test-rule.md

This document already exists in the contribute directory, and the current document can be deleted.

Note: This issue is only for contributors who have not contributed to eventmesh and want to contribute to eventmesh, and can only be claimed once per contributor!

Are you willing to submit PR?

• [ ] Yes I am willing to submit a PR!

[luodanqing]

Yes I am willing to submit a PR!

[qqeasonchen]

@luodanqing sure