TST: fix assert_categorical_equal message
- [x] stage 1 of #13076
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] no whatsnew as it is internal test method

```
a = pd.Categorical([1, 2, 3, 4])
b = pd.Categorical([1, 2, 3, 5])
tm.assert_categorical_equal(a, b)
# Categorical.categories are different
# ...

a = pd.Categorical([1, 2, 4, 3], categories=[1, 2, 3, 4])
b = pd.Categorical([1, 2, 3, 4], categories=[1, 2, 3, 4])
tm.assert_categorical_equal(a, b)
# Categorical.codes values are different (50.0 %)
# ...
```

This must be after #13088.
