tests: make qvm-sync-clock test more reliable

Compare the time with the "current" time retrieved from ClockVM just
before comparing, not with the test start time. This should work even if
the test machine is quite slow (test taking more than 30s).

(cherry picked from commit 3815e0b)

qubes/tests/integ/vm_qrexec_gui.py

@@ -399,10 +399,13 @@ def test_210_time_sync(self):
             self.assertEqual(p.returncode, 0)
             vm_time, _ = self.loop.run_until_complete(
                 self.testvm2.run_for_stdio('date -u +%s'))
-            self.assertAlmostEquals(int(vm_time), int(start_time), delta=30)
+            # get current time
+            current_time, _ = self.loop.run_until_complete(
+                self.testvm1.run_for_stdio('date -u +%s'))
+            self.assertAlmostEquals(int(vm_time), int(current_time), delta=30)
 
             dom0_time = subprocess.check_output(['date', '-u', '+%s'])
-            self.assertAlmostEquals(int(dom0_time), int(start_time), delta=30)
+            self.assertAlmostEquals(int(dom0_time), int(current_time), delta=30)
 
         except:
             # reset time to some approximation of the real time