Since at least 2019:
npm test -- bar.spec.js
In 2015:
In order to run a specific test, you’ll need to use the jest command. npm test will not work. To access jest directly on the command line, install it via npm i -g jest-cli or yarn global add jest-cli.
Then simply run your specific test with jest bar.spec.js.
Note: You don’t have to enter the full path to your test file. The argument is interpreted as a regular expression. Any part of the full path that uniquely identifies a file suffices.