revisions: Handle updating the test state when deleting a result

While we don't have an API to delete a test result at the moment, it's
future proof to fix this straight away. That means we can also use it
when tinkering with the db from manage.py shell for instance.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>

patchwork/models.py

@@ -647,8 +647,10 @@ def refresh_test_state(self):
         results = TestResult.objects.filter(revision=self)
         if results.count() > 0:
             self.test_state = max([r.state for r in results])
-            self.save()
-            self.series.save()
+        else:
+            self.test_state = None
+        self.save()
+        self.series.save()
 
     def human_name(self):
         name = self.series.name