More precise warning detection

Reduce false-positive while detecting warnings by searching for the word
"Warning" at the beginning of a line only.

lib/test/mdx_test.ml

@@ -203,9 +203,11 @@ let rec error_padding = function
       let xs = error_padding xs in
       x :: xs
 
+let contains_warnings l =
+  String.is_prefix ~affix:"Warning" l || String.is_infix ~affix:"\nWarning" l
+
 let eval_ocaml ~(block : Block.t) ?syntax ?root c ppf errors =
   let cmd = block.contents |> remove_padding in
-  let contains_warnings = String.is_infix ~affix:"Warning" in
   let error_lines =
     match eval_test ?root ~block c cmd with
     | Ok lines -> List.filter contains_warnings lines

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

@@ -35,9 +35,6 @@ Test against some false positives:
 ```ocaml
 let x = [ "Warning" ]
 ```
-```mdx-error
-val x : string list = ["Warning"]
-```
 
 ```ocaml
 module Warning = struct
@@ -46,10 +43,6 @@ end
 
 let warning = Warning.Warning
 ```
-```mdx-error
-module Warning : sig type t = Warning end
-val warning : Warning.t = Warning.Warning
-```
 
 Intended false positive: