explain how to run a single ruby test file in CONTRIBUTING.md (#3085)

* explain how to run a single ruby test file in CONTRIBUTING.md

* adds how to run a single ruby test

Author: dorianmariecom

CONTRIBUTING.md

@@ -3,31 +3,50 @@
 1. Install [Yarn](https://yarnpkg.com/)
 
 2. Run the following commands to set up the development environment.
+
 ```
 bundle install
 yarn
 ```
 
 ## Making sure your changes pass all tests
+
 There are a number of automated checks which run on GitHub Actions when a pull request is created.
+
 You can run those checks on your own locally to make sure that your changes would not break the CI build.
 
 ### 1. Check the code for JavaScript style violations
+
 ```
 yarn lint
 ```
 
 ### 2. Check the code for Ruby style violations
+
 ```
 bundle exec rubocop
 ```
 
 ### 3. Run the JavaScript test suite
+
 ```
 yarn test
 ```
 
 ### 4. Run the Ruby test suite
+
 ```
 bundle exec rake test
 ```
+
+#### 4.1 Run a single ruby test file
+
+```
+bundle exec rake test TEST=test/rake_tasks_test.rb
+```
+
+#### 4.2 Run a single ruby test
+
+```
+ruby -I test test/rake_tasks_test.rb -n test_rake_webpacker_install
+```