Use warning 9 instead of 6 in test

(warning 6 is enabled by default in 4.13+)

Author: emillon

test/bin/mdx-test/expect/warnings/test-case.md

@@ -1,34 +1,31 @@
 No warning is printed by default:
 
 ```ocaml
-let () =
-  let f ~x:() = () in
-  f ();;
-let x = 4
+type p = { x : int ; y : int }
+
+let x { x } = x
 ```
 
 Warning attributes must be set to print them:
 
 ```ocaml version<4.12
-[@@@warning "+6"]
-let () =
-  let f ~x:() = () in
-  f ();;
-let x = 4
+[@@@warning "+9"]
+let x { x } = x
 ```
 ```mdx-error
 ...
-Warning 6: label x was omitted in the application of this function.
+Warning 9: the following labels are not bound in this record pattern:
+y
+Either bind these labels explicitly or add '; _' to the pattern.
 ```
 
 ```ocaml version>=4.12
-[@@@warning "+6"]
-let () =
-  let f ~x:() = () in
-  f ();;
-let x = 4
+[@@@warning "+9"]
+let x { x } = x
 ```
 ```mdx-error
-Line 4, characters 5-6:
-Warning 6 [labels-omitted]: label x was omitted in the application of this function.
+Line 2, characters 9-14:
+Warning 9 [missing-record-field-pattern]: the following labels are not bound in this record pattern:
+y
+Either bind these labels explicitly or add '; _' to the pattern.
 ```