Fix alcotest constraint
Fix alcotest constraint

although it seems the testsuite won't pass anyway:

$ opam reinstall -t irmin-unix.1.3.3 -y alcotest.0.8.5

 #=== ERROR while compiling irmin-unix.1.3.3 ===================================#
 # context     2.0.6 | linux/x86_64 | ocaml-base-compiler.4.05.0 | https://opam.ocaml.org#1d8fdb21
 # path        ~/.opam/4.05.0+test/.opam-switch/build/irmin-unix.1.3.3
 # command     ~/.opam/opam-init/hooks/sandbox.sh build jbuilder runtest -p irmin-unix
 # exit-code   1
 # env-file    ~/.opam/log/irmin-unix-20844-e0ba4f.env
 # output-file ~/.opam/log/irmin-unix-20844-e0ba4f.out
 ### output ###
 # [...]

 # File "test/common/test_store.ml", line 90, characters 24-27:
 # Error: This expression has type Graph.node = S.Tree.Hash.t
 #        but an expression was expected of type S.Tree.hash

dbuenzli/opam-repository,mjambon/opam-repository,Chris00/opam-repository,kkirstein/opam-repository,toots/opam-repository,lefessan/opam-repository,nberth/opam-repository,Chris00/opam-repository,talex5/opam-repository,astrada/opam-repository,jonludlam/opam-repository,cakeplus/opam-repository,pirbo/opam-repository,pqwy/opam-repository,yallop/opam-repository,astrada/opam-repository,hannesm/opam-repository,yallop/opam-repository,Leonidas-from-XIV/opam-repository,kkirstein/opam-repository,lefessan/opam-repository,seliopou/opam-repository,arlencox/opam-repository,codinuum/opam-repository,emillon/opam-repository,jhwoodyatt/opam-repository,ocaml/opam-repository,toots/opam-repository,codinuum/opam-repository,hannesm/opam-repository,djs55/opam-repository,c-cube/opam-repository,hakuch/opam-repository,ocaml/opam-repository,dbuenzli/opam-repository,Leonidas-from-XIV/opam-repository,pqwy/opam-repository,seliopou/opam-repository,abate/opam-repository,nberth/opam-repository,mjambon/opam-repository,emillon/opam-repository,andersfugmann/opam-repository,djs55/opam-repository,andersfugmann/opam-repository,pirbo/opam-repository,jonludlam/opam-repository,talex5/opam-repository,orbitz/opam-repository,jhwoodyatt/opam-repository,fpottier/opam-repository,c-cube/opam-repository,fpottier/opam-repository