Add a qcheck upper bound for preface.1.0.0

[jmid]

preface.1.0.0 is incompatible with qcheck.0.24 as observed in #27468. This PR therefore adds an upper bound.

In more detail: qcheck.0.24 adding missing result combinators in c-cube/qcheck#308 which cause a signature clash with the handwritten result combinator already present in preface.1.0.0
https://github.com/xvw/preface/blob/b7f5210830b7916033c56ede6108c2c439c31cdf/test/preface_qcheck/gen.ml#L66-L68 and thus resulting in the error message:

# Error: The implementation test/preface_qcheck/gen.ml
#        does not match the interface test/preface_qcheck/.preface_qcheck.objs/byte/preface_qcheck__Gen.cmi:
#         Values do not match:
#           val result :
#             ?distribution:float ->
#             (Random.State.t -> 'a) ->
#             (Random.State.t -> 'b) -> Random.State.t -> ('a, 'b) result
#         is not included in
#           val result : ?ratio:float -> 'a t -> 'e t -> ('a, 'e) result t
#         The type
#           ?distribution:float ->
#           (Random.State.t -> 'a) ->
#           (Random.State.t -> 'b) -> Random.State.t -> ('a, 'b) result
#         is not compatible with the type
#           ?ratio:float -> 'c t -> 'd t -> ('c, 'd) result t
#         File "test/preface_qcheck/gen.mli", line 61, characters 8-56:
#           Expected declaration
#         File "test/preface_qcheck/gen.ml", line 66, characters 4-10:
#           Actual declaration

To confirm that this PR indeed fixes the above, I'll be happy to wait and rebase it on master once #27468 is merged.

[jmid]

There's one failure on opam.2.0 running ocaml.4.14, due to a timeout:

/home/opam: (run (network host)
                 (shell "(opam depext --with-test preface.1.0.0 && opam reinstall --with-test preface.1.0.0) || true"))
# Detecting depexts using vars: arch=x86_64, os=linux, os-distribution=debian, os-family=debian
# No extra OS packages requirements found.
# All required OS packages found.
[ERROR] Sorry, resolution of the request timed out.
        Try to specify a simpler request, use a different solver, or increase the allowed time by setting OPAMSOLVERTIMEOUT to a bigger value (currently, it is set to 60.0 seconds).

[raphael-proust]

thanks ❤️