Use cmp in tests

A conventional pattern in tests is to use the [cmp package](https://pkg.go.dev/github.com/google/go-cmp/cmp) and the following pattern:

```
if diff := cmp.Diff(got, want); diff != "" {
  t.Errorf("<descriptive error message referring to got and want>")
```

Ideally this should be followed in all tests and much as possible. 