run-test: avoid byte issue when replacing output file of python test

Otherwise we get error like::

  FileNotFoundError: [Errno 2] No such file or directory: "b'…/tests/test-minirst.py'.out"

Differential Revision: https://phab.mercurial-scm.org/D9868

tests/run-tests.py

@@ -2278,7 +2278,7 b' class TestResult(unittest._TextTestResul'
                         if test.path.endswith(b'.t'):
                             rename(test.errpath, test.path)
                         else:
-                            rename(test.errpath, '%s.out' % test.path)
+                            rename(test.errpath, b'%s.out' % test.path)
                         accepted = True
             if not accepted:
                 self.faildata[test.name] = b''.join(lines)