This test checks whether the non-deterministic mode works with the `dune` `mdx`
stanza.

Deterministic stanzas should get run and corrected, as for 1 plus one is not 3:

```ocaml
# 1 + 1;;
- : int = 42
```

Non-deterministic ones should not be updated, since whatever `Random` outputs
should be random:

<!-- $MDX non-deterministic=command -->
```ocaml
# Random.int 1000;;
- : int = 42
```
