We use simplecov to check our test suite coverage. To do so, run your test suite with COVERAGE=true and then open the index.html located in /coverage/ in a browser. It's always informative to check the coverage of your newly written code before opening a PR.
By default, Chrome is run in headless mode. If you want to see what happens in the browser, you can set the environment variable HEADLESS_TEST to false and use a VNC client (on a Mac, use Finder > Go > Connect to server...vnc://0.0.0.0:5900), with password secret.
To improve the speed of running specs, you can run the specs parallelized.
First, create the databases and load schema with rails parallel:create parallel:load_schema.
Run the specs with rails parallel:spec.