log: don't create foo.log

test_tempest_set_log_file was calling tempest_set_log_file and never
resetting this value. That caused some other test to create foo.log
and not clean it up.

Change-Id: I23efe8fb89531752b01bdf9c0fbdc39c77058a51

oslo_log/tests/unit/test_log.py

@@ -800,6 +800,7 @@ class SetDefaultsTestCase(BaseTestCase):
     def test_tempest_set_log_file(self):
         log_file = 'foo.log'
         log.tempest_set_log_file(log_file)
+        self.addCleanup(log.tempest_set_log_file, None)
         log.set_defaults()
         self.conf([])
         self.assertEqual(log_file, self.conf.log_file)