ci: verbose output when utf8 on win test fails

.github/workflows/tests.yml

@@ -122,4 +122,5 @@ jobs:
         # Warning: the diff line below is PowerShell syntax, not bash!
         run: |
           echo ćś | readalongs make-xml -l fra - - | findstr /v meta > cs.readalong
+          diff (cat cs.readalong) (cat test/data/cs-ref.readalong)
           if (diff (cat cs.readalong) (cat test/data/cs-ref.readalong)) { throw "Output did not match reference" }