When working on a patch for DevTools, there’s almost never a reason not to add a new test. If you are fixing a bug, you probably should write a new test to prevent this bug from occurring again. If you’re implementing a new feature, you should write new tests to cover the aspects of this new feature.
Are there enough tests for my patch?
Are they the right types of tests?
We use three suites of tests:
Chrome mochitests: Unit-test style of tests, but with a browser window. Mostly testing APIs that interact with the DOM.
DevTools mochitests: Integration style of tests. Fires up a whole browser window with every test and you can test clicking on buttons, etc.
More information about the different types of tests can be found on the automated testing page at MDN.
To run all DevTools tests, regardless of suite type:
./mach test devtools/*
Have a look at the child pages for more specific commands for running only a single suite or single test in a suite.