tests: avoid implicit bytes -> unicode -> bytes roundtrip on Python 2

Before the change, there were two implicit conversions:

* The format string was converted to unicode because the substituted values were
  unicode.
* When printing the message in log(), the string was converted from unicode to
  str.

tests/run-tests.py

@@ -1165,10 +1165,7 b' class Test(unittest.TestCase):'
         if self._keeptmpdir:
             log(
                 '\nKeeping testtmp dir: %s\nKeeping threadtmp dir: %s'
-                % (
-                    self._testtmp.decode('utf-8'),
-                    self._threadtmp.decode('utf-8'),
-                )
+                % (_bytes2sys(self._testtmp), _bytes2sys(self._threadtmp),)
             )
         else:
             try: