remove the circular io-page -> mirage-types-lwt test-only dependency

This is only here for {test}, but it's still problematic for OPAM 1.1
clients.  It also appears to happen sometimes on OPAM 1.2 (#3128)
which may be a bug (but depends on the semantics of test).

Fixes #3128
Merge pull request #3129 from avsm/master

remove the circular io-page -> mirage-types-lwt test-only dependency