Fix tests (add dependency on alcotest)

Author: mtelvers

packages/dockerfile/dockerfile.8.2.2/opam

@@ -36,6 +36,7 @@ depends: [
   "fmt" {>= "0.8.7"}
   "ppx_sexp_conv" {>= "v0.9.0"}
   "sexplib"
+  "alcotest" {>= "1.7.0" & with-test}
   "odoc" {with-doc}
 ]
 conflicts: [