Test case: Don't rely on the output of 'nice 0'

Author: jonludlam

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

@@ -2,12 +2,12 @@ Mdx can skip blocks based on `os_type`:
 
 ```ocaml os_type<>Win32
 # #require "unix"
-# Unix.nice 0
-- : int = 0
+# ignore (Unix.nice 0)
+- : unit = ()
 ```
 
 ```ocaml os_type=Win32
 # #require "unix"
-# Unix.nice 0
+# ignore (Unix.nice 0)
 Exception: Invalid_argument "Unix.nice not implemented".
 ```