add sub function

main_test.go

@@ -20,3 +20,20 @@ func Test_sum(t *testing.T) {
 		}
 	}
 }
+
+func Test_sub(t *testing.T) {
+	tests := []struct {
+		a, b int
+		want int
+	}{
+		{1, 2, -1},
+		{-1, 1, -2},
+		{0, 0, 0},
+	}
+
+	for _, test := range tests {
+		if got := sub(test.a, test.b); got != test.want {
+			t.Errorf("sub(%d, %d) = %d, want %d", test.a, test.b, got, test.want)
+		}
+	}
+}