Sabrina, help me understand—are you suggesting that the process of design doesn't need any rigour because it's going to be tested anyway? Because your response seems to suggest that.
As a process, design is only as good as the clarity that prompts it. A designer needs to start with a clear understanding of the problem and its human context.
Testing only exists to confirm that the design—and the interpretation that shaped it—is correct when in context—or to highlight deficiencies for further development.