pa_incr tests are not compatible with ocaml 5.1

```
File "README.md", line 1, characters 0-0:
\# /usr/bin/git --no-pager diff --no-index --color=always -u _build/default/README.md _build/default/.mdx/README.md.corrected
\# diff --git a/_build/default/README.md b/_build/default/.mdx/README.md.corrected
\# index 484ed8d..510b599 100644
\# --- a/_build/default/README.md
\# +++ b/_build/default/.mdx/README.md.corrected
\# @@ -404,10 +404,11 @@ We can see `filter` does indeed work as expected.
\#
\#  ```ocaml
\#  # let var_list = to_var_list [2;3;5]
\# -val var_list : _[> `Cons of int * 'a | `Nil ] Var.t as 'a = <abstr>
\# +val var_list : ([> `Cons of int * 'a | `Nil ] as '_weak1) Var.t as 'a =
\# +  <abstr>
\#
\#  # let incr_list = to_incr_list var_list
\# -val incr_list : _[> `Cons of int * 'a | `Nil ] t as 'a = <abstr>
\# +val incr_list : ([> `Cons of int * 'a | `Nil ] as '_weak2) t as 'a = <abstr>
\#
\#  # let res_list = filter (fun x -> x mod 2 = 1)
```
Signed-off-by: Marcello Seri <[email protected]>

Author: mseri

packages/par_incr/par_incr.0.1/opam

@@ -30,7 +30,7 @@ build: [
     "-j"
     jobs
     "@install"
-    "@runtest" {with-test}
+    "@runtest" {with-test & ocaml:version < "5.1"}
     "@doc" {with-doc}
   ]
 ]