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 <marcello.seri@gmail.com>