Test Apex Classes

1. Asks for approval of a plan before running tests. Approving the plan is one of the steps most often left to the user.
2. Runs the requested test classes or methods in the sandbox and reports which passed, failed or were skipped.
3. Reports skipped tests and why, for example test methods that do not exist on the org yet.
4. Checks whether the test script has changed since an earlier run and what extra test classes the newer version needs.
5. Retrieves a test class from the org, changes it, deploys it and rechecks coverage.
6. Prepares a clean branch holding only the test change for a pull request.

When tests fail, the next step is Diagnose Apex Errors. Writing or fixing the classes themselves is covered by Engineer Apex Classes.