run-test: use byte for 'common-pattern.py' path

This is required for Python 3.

tests/run-tests.py

@@ -969,7 +969,7 b' class Test(unittest.TestCase):'
         r.append((self._escapepath(self._testtmp), b'$TESTTMP'))
 
         testdir = os.path.dirname(self.path)
-        replacementfile = os.path.join(testdir, 'common-pattern.py')
+        replacementfile = os.path.join(testdir, b'common-pattern.py')
 
         if os.path.exists(replacementfile):
             data = {}