FI[R]ST: Good Tests Should Be [R]epeatable
Tests don’t appear out of thin air—you’re the one who gets to design them, which means they are entirely under your control. You have the power to devise a test’s conditions, which also means that you don’t need a crystal ball to know what the test outcome should be. Part of your job in test design, then, is to provide an assertion that specifies what the outcome should be each and every time the test is run.
A repeatable test is one that produces the same results each time you run it. To accomplish repeatable tests, you must isolate them from anything in the external environment not under your direct control.
Your system will inevitably need to interact with elements not under your control, however. ...