Add docs for debugging Jest tests

doc/development/testing_guide/frontend_testing.md

@@ -26,6 +26,10 @@ It is not yet a requirement to use Jest. You can view the
 [epic](https://gitlab.com/groups/gitlab-org/-/epics/873) of issues
 we need to solve before being able to use Jest for all our needs.
+### Debugging Jest tests
+
+Running `yarn jest-debug` will run Jest in debug mode, allowing you to debug/inspect as described in the [Jest docs](https://jestjs.io/docs/en/troubleshooting#tests-are-failing-and-you-don-t-know-why).
+
 ### Timeout error
 The default timeout for Jest is set in

package.json

@@ -7,6 +7,7 @@
     "eslint-fix": "eslint --max-warnings 0 --report-unused-disable-directives --ext .js,.vue --fix .",
     "eslint-report": "eslint --max-warnings 0 --ext .js,.vue --format html --output-file ./eslint-report.html --no-inline-config .",
     "jest": "BABEL_ENV=jest jest",
+    "jest-debug": "BABEL_ENV=jest node --inspect-brk node_modules/.bin/jest --runInBand",
     "jsdoc": "jsdoc -c config/jsdocs.config.js",
     "karma": "BABEL_ENV=${BABEL_ENV:=karma} karma start --single-run true config/karma.config.js",
     "karma-coverage": "BABEL_ENV=coverage karma start --single-run true config/karma.config.js",