lwt_eio <= 0.3 tests fail with recent mdx

Due to the extra verbose output
```
\# File "test/test.md", line 1, characters 0-0:
\# /usr/bin/git --no-pager diff --no-index --color=always -u _build/default/test/test.md _build/default/test/.mdx/test.md.corrected
\# diff --git a/_build/default/test/test.md b/_build/default/test/.mdx/test.md.corrected
\# index 1245303..88fb312 100644
\# --- a/_build/default/test/test.md
\# +++ b/_build/default/test/.mdx/test.md.corrected
\# @@ -42,19 +42,19 @@ during each iteration of the main loop, so the Lwt thread runs twice as often.
\#      );;
\#  +eio: i = 1
\#  +  lwt: i = 1
\# -+eio: i = 2
\#  +  lwt: i = 2
\# ++eio: i = 2
\#  +  lwt: i = 3
\#  +eio: i = 3
\#  +  lwt: i = 4
\# -+  lwt: i = 5
\#  +eio: i = 4
\# -+  lwt: i = 6
\# -+  lwt: i = 7
\# ++  lwt: i = 5
\#  +eio: i = 5
\# -+  lwt: i = 8
\# ++  lwt: i = 6
\#  +eio: i = 6
\# ++  lwt: i = 7
\#  +eio: i = 7
\# ++  lwt: i = 8
\#  +eio: i = 8
```

Signed-off-by: Marcello Seri <[email protected]>

Author: mseri

packages/lwt_eio/lwt_eio.0.2/opam

@@ -12,7 +12,7 @@ depends: [
   "dune" {>= "2.9"}
   "eio" {>= "0.2" & < "0.11"}
   "lwt"
-  "mdx" {>= "1.10.0" & with-test}
+  "mdx" {>= "1.10.0" & < "2.3.1" & with-test}
   "eio_main" {with-test}
   "odoc" {with-doc}
 ]

packages/lwt_eio/lwt_eio.0.3/opam

@@ -12,7 +12,7 @@ depends: [
   "dune" {>= "2.9"}
   "eio" {>= "0.7" & < "0.11"}
   "lwt"
-  "mdx" {>= "1.10.0" & with-test}
+  "mdx" {>= "1.10.0" & < "2.3.1" & with-test}
   "eio_main" {with-test}
   "odoc" {with-doc}
 ]