There are several methods on *testing.T for reporting test failures. You’ve already seen Error, which builds a failure description string out of a comma-separated list of values.
If you’d rather use a Printf-style formatting string to generate your message, use the Errorf method instead:
t.Errorf("incorrect result: expected %d, got %d", 5, result)
While Error and Errorf mark a test as failed, the test function continues running. If you think a test function should stop processing as soon as a failure is found, use the Fatal and Fatalf methods. The Fatal method works like Error, and the Fatalf method works like Errorf. The difference is that the test function exits immediately after the test failure message is generated. Note that Fatal only ends the current test; any remaining test functions will execute after the current test function exits.
When should you use Fatal/Fatalf and when should you use Error/Errorf? If the failure of a check in a test means that further checks in the same test function will always fail or cause the test to panic, use Fatal or Fatalf. If you are testing several independent items (such as validating fields in a struct), then use Error or Errorf so you can report many problems at once. This makes it easier to fix multiple problems without rerunning your tests over and over.