tests: fix race condition in firewall formatting test

Use fixed timestamp.

qubes/tests/vm/qubesvm.py

@@ -1278,11 +1278,12 @@ class TC_90_QubesVM(QubesVMTestsMixin, qubes.tests.QubesTestCase):
             '/qubes-service/meminfo-writer': '1',
         })
 
+    @unittest.mock.patch('datetime.datetime')
     @unittest.mock.patch('qubes.utils.get_timezone')
     @unittest.mock.patch('qubes.utils.urandom')
     @unittest.mock.patch('qubes.vm.qubesvm.QubesVM.untrusted_qdb')
     def test_621_qdb_vm_with_network(self, mock_qubesdb, mock_urandom,
-            mock_timezone):
+            mock_timezone, mock_datetime):
         mock_urandom.return_value = b'A' * 64
         mock_timezone.return_value = 'UTC'
         template = self.get_vm(cls=qubes.vm.templatevm.TemplateVM, name='template')
@@ -1300,6 +1301,8 @@ class TC_90_QubesVM(QubesVMTestsMixin, qubes.tests.QubesTestCase):
         mock_qubesdb.write.side_effect = test_qubesdb.write
         mock_qubesdb.rm.side_effect = test_qubesdb.rm
         self.maxDiff = None
+        mock_datetime.now.returnvalue = \
+            datetime.datetime(2019, 2, 27, 15, 12, 15, 385822)
 
         iptables_header = (
             '# Generated by Qubes Core on {}\n'