mirage: expect tests only good with 4.x

Author: hannesm

packages/mirage/mirage.4.9.0/opam

@@ -19,6 +19,7 @@ build: [
 
 depends: [
   "ocaml" {>= "4.13.0"}
+  "ocaml" {with-test & < "5"}
   "dune" {>= "2.9.0"}
   "astring"
   "cmdliner" {>= "1.2.0"}