add documentation for karma testing with specific files/folders

doc/development/testing_guide/frontend_testing.md

@@ -153,7 +153,7 @@ is sufficient (and saves you some time).
 
 While developing locally, it may be helpful to keep karma running so that you
 can get instant feedback on as you write tests and modify code. To do this
-you can start karma with `npm run karma-start`.  It will compile the javascript
+you can start karma with `yarn run karma-start`. It will compile the javascript
 assets and run a server at `http://localhost:9876/` where it will automatically
 run the tests on any browser which connects to it. You can enter that url on
 multiple browsers at once to have it run the tests on each in parallel.
@@ -165,6 +165,20 @@ excluded tests (with `fdescribe` or `xdescribe`) to get karma to run only the
 tests you want while you're working on a specific feature, but make sure to
 remove these directives when you commit your code.
 
+It is also possible to only run karma on specific folders or files by simply
+appending the file name to the karma command:
+
+```bash
+# Run all files
+yarn karma-start
+# Run specific spec files
+yarn karma-start profile/account/components/update_username_spec.js
+# Run specific spec folder
+yarn karma-start profile/account/components/
+# Run all specs in vue_shared and vue_mr_widget
+yarn karma-start /vue_shared/ /vue_mr_widget/
+```
+
 ## RSpec feature integration tests
 
 Information on setting up and running RSpec integration tests with