Testing is not about making sure that test cases pass. It’s about using any means to find problems that harm or annoy people.
Testing involves far more than checking to see that the program returns a functionally correct result from a calculation.
Testing means putting something to the test, investigating and learning about it through experimentation, interaction, and challenge. Yes, tools may help in important ways, but the point is to discover how the product serves human purposes, and how it might miss the mark.
So a skilled tester does not ask simply “Does this check pass or fail?” Instead, the skilled tester probes the product and asks a much more rich and fundamental question: Is there a problem here?