OPAM: add with-test for ff-pbt dependency
Add constraint with-test for ff-pbt in the OPAM definition. It is fixed in
ocaml/opam-repository for bls12-381.6.0.1, but not in the release tag here.
There is therefore an inconsistency between the OPAM file definitions. As it is
not related to the code, I suggest to leave it like this.
Edited by Danny Willems