Add test runner instructions to CONTRIBUTING.md

The test command and Node version requirement were only documented
in CI workflow YAML, making it hard for new contributors to discover
how to run tests locally.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: vaind

danger/CONTRIBUTING.md

@@ -1,5 +1,16 @@
 # Contributing
 
+## Running tests
+
+The test suite uses Node.js's built-in test runner (requires Node 18+):
+
+```sh
+cd danger
+node --test
+```
+
+No dependencies to install — the tests use only `node:test` and `node:assert`.
+
 ## How to run dangerfile locally
 
 - [Working on your Dangerfile](https://danger.systems/js/guides/the_dangerfile.html#working-on-your-dangerfile)