eio_main tests are not compatible with mdx 2.3.1

Fail with extra verbose output:
```
\#=== ERROR while compiling eio_main.0.6 =======================================#
\# context              2.2.0~alpha3~dev | linux/x86_64 | ocaml-base-compiler.5.1.0 | file:///home/opam/opam-repository
\# path                 ~/.opam/5.1/.opam-switch/build/eio_main.0.6
\# command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p eio_main -j 255 --promote-install-files=false @install @runtest
\# exit-code            1
\# env-file             ~/.opam/log/eio_main-7-67a807.env
\# output-file          ~/.opam/log/eio_main-7-67a807.out
\### output ###
\# File "tests/stream.md", line 1, characters 0-0:
\# /usr/bin/git --no-pager diff --no-index --color=always -u _build/default/tests/stream.md _build/default/tests/.mdx/stream.md.corrected
\# diff --git a/_build/default/tests/stream.md b/_build/default/tests/.mdx/stream.md.corrected
\# index ab34c10..fcb1c92 100644
\# --- a/_build/default/tests/stream.md
\# +++ b/_build/default/tests/.mdx/stream.md.corrected
\# @@ -48,6 +48,7 @@ Simple non-blocking case
\#  +Got 1 from stream
\#  +Reading from stream
\#  +Got 2 from stream
\# +
\#  - : unit = ()
[...]
```
Signed-off-by: Marcello Seri <[email protected]>

Author: mseri

packages/eio_main/eio_main.0.6/opam

@@ -10,7 +10,7 @@ bug-reports: "https://github.com/ocaml-multicore/eio/issues"
 depends: [
   "dune" {>= "2.9"}
   "eio_linux" {= version & os = "linux"}
-  "mdx" {>= "1.10.0" & with-test}
+  "mdx" {>= "1.10.0" & < "2.3.1" & with-test}
   "eio_luv" {= version}
   "odoc" {with-doc}
 ]

packages/eio_main/eio_main.0.7/opam

@@ -10,7 +10,7 @@ bug-reports: "https://github.com/ocaml-multicore/eio/issues"
 depends: [
   "dune" {>= "2.9"}
   "eio_linux" {= version & os = "linux"}
-  "mdx" {>= "1.10.0" & with-test}
+  "mdx" {>= "1.10.0" & < "2.3.1" & with-test}
   "eio_luv" {= version}
   "odoc" {with-doc}
 ]

packages/eio_main/eio_main.0.8.1/opam

@@ -10,7 +10,7 @@ bug-reports: "https://github.com/ocaml-multicore/eio/issues"
 depends: [
   "dune" {>= "3.0"}
   "eio_linux" {= version & os = "linux"}
-  "mdx" {>= "1.10.0" & with-test}
+  "mdx" {>= "1.10.0" & < "2.3.1" & with-test}
   "eio_luv" {= version}
   "odoc" {with-doc}
 ]

packages/eio_main/eio_main.0.9/opam

@@ -9,7 +9,7 @@ doc: "https://ocaml-multicore.github.io/eio/"
 bug-reports: "https://github.com/ocaml-multicore/eio/issues"
 depends: [
   "dune" {>= "3.7"}
-  "mdx" {>= "2.2.0" & with-test}
+  "mdx" {>= "2.2.0" & < "2.3.1" & with-test}
   "eio_linux" {= version & os = "linux"}
   "eio_posix" {= version & os != "win32"}
   "eio_luv" {= version & (os = "win32" | with-test)}