Subtests with t.Run

Use t.Run to group related tests and enable selective execution.

CORRECT

func Test_User_Validation(t *testing.T) {
    t.Run("valid email", func(t *testing.T) {
        user := User{Email: "test@example.com"}
        if err := user.Validate(); err != nil {
            t.Errorf("expected valid, got error: %v", err)
        }
    })