Update mdx test for new warning policy

Signed-off-by: Ulysse Gérard <thevoodoos@gmail.com>

test/blackbox-tests/test-cases/mdx-stanza/run.t

@@ -12,9 +12,16 @@ It also requires dune lang 2.4 or higher
   File "dune-project", line 3, characters 11-14:
   3 | (using mdx 0.1)
                  ^^^
-  Error: Version 0.1 of mdx is not supported until dune lang 2.4.
-  There are no supported versions of this extension for dune lang 2.3.
+  Warning: Version 0.1 of mdx extension to verify code blocks in .md files is
+  not supported until version 2.4 of the dune language.
+  There are no supported versions of this extension in version 2.3 of the dune
+  language.
 
+  File "dune", line 1, characters 0-5:
+  1 | (mdx)
+      ^^^^^
+  Error: 'mdx' is only available since version 2.4 of the dune language. Please
+  update your dune-project file to have (lang 2.4).
   [1]
 
 You can use the mdx stanza to check your documentation in markdown and mli files