software/erp5/test: fix flaky test_activity_processing

wait a first time that ERP5 have finished processing activities for
initial site creation and tolerate that during this time zopes might
reply with error code

software/erp5/test/test/test_erp5.py

@@ -802,29 +802,39 @@ class ZopeTestMixin(CrontabMixin):
     self.assertIn('Thread ', dump_response.text)
 
   def test_activity_processing(self):
-    requests.get(
-        self.zope_verify_activity_processing_url,
-        params={
-            'mode': 'activate'
-        },
-        verify=False,
-    ).raise_for_status()
-
-    for retry in range(60):
+    def wait_for_activities(max_retries):
+      for retry in range(max_retries):
         time.sleep(10)
-      count = requests.get(
+        resp = requests.get(
             self.zope_verify_activity_processing_url,
             params={
                 'mode': 'count',
                 'retry': retry,
             },
             verify=False,
-      ).json()['count']
+        )
+        if not resp.ok:
+          # XXX we start by flushing existing activities from site creation
+          # and inital upgrader run. During this time it may happen that
+          # ERP5 replies with site errors, we tolerate these errors and only
+          # check the final state.
+          continue
+        count = resp.json()['count']
         if not count:
           break
       else:
         self.assertEqual(count, 0)
 
+    wait_for_activities(60)
+    requests.get(
+        self.zope_verify_activity_processing_url,
+        params={
+            'mode': 'activate'
+        },
+        verify=False,
+    ).raise_for_status()
+    wait_for_activities(10)
+
 
 class TestZopeMedusa(ZopeTestMixin, ERP5InstanceTestCase):
   wsgi = False