fix eio.1.3+ox on latest oxcaml

[avsm]

We need to explicitly curry a function application in eio_mock (avsm/eio@7de26f5) or oxcaml complains about this with this error:

Error: This application is complete, but surplus arguments were provided afterwards.
           When passing or calling local values, extra arguments are passed in a separate application.
Hint: Try wrapping the marked application in parentheses.