make a compareString function for goada_test.go

@lemire @anonrig  hi  add helper functions to prevent this code repetition? For example, if I create a helper function called compareString like 

this: 

```go
func compareString(t *testing.T, expected, actual, message string) {
	if expected != actual {
		t.Errorf("Expected %s, but got %s. %s", expected, actual, message)
	}
}
```