Test Runner is a framework for running automatic tests, which are helpful if you want to ensure important components of your application still work as they used to in the past ("regression testing").
You can define sets of tests ("Test Suites"), each containing several tests of parts of your application ("Test Cases"), and then run all of them or any subset of them. As you run the tests, you can see which test is currently running, and for each test whether it has finished successfully or not.
For more details about this new feature, including a short tutorial, see the documentation page.
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