Show the failing string when conversion fails in MBConvTestCase.

No real changes, just give more information when a test fails in
MBConvTestCase::TestDecoder().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65742 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/mbconv/mbconvtest.cpp

@@ -1040,7 +1040,12 @@ void MBConvTestCase::TestDecoder(
         0
         );
     // make sure the correct output length was calculated
-    CPPUNIT_ASSERT_EQUAL( wideChars, outputWritten );
+    WX_ASSERT_EQUAL_MESSAGE
+    (
+        ("while converting \"%s\"", multiBuffer),
+        wideChars,
+        outputWritten
+    );
 
     // convert the string
     size_t guardChars = 8; // to make sure we're not overrunning the output buffer