Fix predicate typo that disabled five bisimulation operations

[matthiasgoergens]

core/test/test_doubly_linked_bisimulation.ml defines its test predicate as:

let pred = function
  | Even -> fun n -> n mod 0 = 0
  | Odd -> fun n -> n mod 0 = 1

n mod 0 raises Division_by_zero for every n. It looks like a typo for n mod 2.

Five operations take that predicate — Exists, Filter_inplace, For_all, Find_elt and Find. On a non-empty list both Doubly_linked and the Foil reference raise, and the simulation loop treats a mutual raise as agreement, so the step is skipped rather than compared. On an empty list the predicate is never reached and they pass trivially. Either way, those five operations have not been testing anything.

How it was found

While surveying hand-written bisimulation tests in OCaml, as background for tapecheck, a port of Hypothesis's choice-tape shrinking engine to base_quickcheck. The pattern being looked for was tests that silently narrow what they can detect.

Happy to drop this if the predicate was deliberate for some reason I have missed.