tests: fix style issue of importing order in test-lock.py

test-lock.py is excluded in test-check-module-imports.t, because
import-checker.py reports that some stdlibs are imported after local
silenttestrunner module.

I can not found out any reason why tests/silenttestrunner.py should be
imported before some stdlibs from the point of functionality view.

tests/test-lock.py

@@ -2,11 +2,12 b' from __future__ import absolute_import'
 
 import copy
 import errno
-import silenttestrunner
 import tempfile
 import types
 import unittest
 
+import silenttestrunner
+
 from mercurial import (
     encoding,
     error,