Docs: Adds tests requirement to bugs checklist (#18576)

* Docs: Adds tests requirement to bugs checklist

* Docs: Adds test for bugs as a requirment

Co-Authored-By: gotjosh <josue@grafana.com>

CONTRIBUTING.md

@@ -48,6 +48,7 @@ To setup a local development environment we recommend reading [Building Grafana
 #### Bug specific checks
 
 - [ ] The pull request contains `Closes: #Issue` or `Fixes: #Issue` in pull request description.
+- [ ] The Pull Request adds tests that replicate the fixed bug and helps avoid regressions.
 
 ### Frontend specific checks