Improve test failure error message

Sometimes this test fails with 4 conversion failurs instead of
3 and we do not know why because there is no useful output.
This change adds the output from the conversions on failure
and adjusts the test to compare the okay and failed count
at the same time by using a tuple.

tests/test_translate_header.py

@@ -84,17 +84,25 @@ def test_translate_header_fails(self):
                     [TESTDATA], r"^.*yaml$", 0, False, outstream=out, errstream=err, output_mode="none"
                 )
 
-                lines = self._readlines(out)
-                self.assertEqual(len(lines), len(failed))
-                self.assertTrue(lines[0].startswith("Failure processing"), f"Line: '{lines[0]}'")
-                self.assertIn("not a mapping", lines[0], f"Line: '{lines[0]}'")
-
-                lines = self._readlines(err)
-                self.assertEqual(len(lines), 13)
-                self.assertTrue(lines[0].startswith("Analyzing"), f"Line: '{lines[0]}'")
-
-        self.assertEqual(len(okay), 10)
-        self.assertEqual(len(failed), 3)
+                out_lines = self._readlines(out)
+                self.assertEqual(len(out_lines), len(failed))
+                self.assertTrue(out_lines[0].startswith("Failure processing"), f"Line: '{out_lines[0]}'")
+                self.assertIn("not a mapping", out_lines[0], f"Line: '{out_lines[0]}'")
+
+                err_lines = self._readlines(err)
+                self.assertEqual(len(err_lines), 13)  # The number of files analyzed
+                self.assertTrue(err_lines[0].startswith("Analyzing"), f"Line: '{err_lines[0]}'")
+
+        # Form message to issue if the test fails.
+        newline = "\n"  # f-string can not accept \ in string.
+        msg = f"""Converted successfully:
+{newline.join(okay)}
+Failed conversions:
+{newline.join(failed)}
+Standard output:
+{newline.join(out_lines)}
+"""
+        self.assertEqual((len(okay), len(failed)), (10, 3), msg=msg)
 
     def test_translate_header_traceback(self):
         """Translate some header files that fail and trigger traceback."""