Fixing broken tests

Frustratingly, this required some annoying changes to `VariableManager`,
so it can now semi-smoothly support breed-changing.

P.S. NetLogo, I love you.

```
breed [cats]
breed [dogs]
breed [rats]

cats-own [x]
dogs-own []
rats-own [x]

O> create-cats 1
T> set x 5
T> x => 5

T> set breed rats
T> x => 5

T> set breed dogs
T> set breed rats
T> x => 0
```

Bravo, NetLogo.  Bravo.