tests: fix run-tests XML reporting on Python 3

cdatasafe wants to work in terms of bytes, but of course we have a
unicode. Easy to work around, especially since we know we'll get utf-8
at the end.

tests/run-tests.py

@@ -2194,7 +2194,8 b' class TextTestRunner(unittest.TextTestRu'
             # the skip message as a text node instead.
             t = doc.createElement('testcase')
             t.setAttribute('name', tc.name)
-            message = cdatasafe(message).decode('utf-8', 'replace')
+            binmessage = message.encode('utf-8')
+            message = cdatasafe(binmessage).decode('utf-8', 'replace')
             cd = doc.createCDATASection(message)
             skipelem = doc.createElement('skipped')
             skipelem.appendChild(cd)