integration/test_storage: Support LXD 4.0 API on tests

In LXD 4.0, responses from `/1.0/storage-pools/default/volumes/custom/vol/snapshots?recursion=1` included a null `expires_at` field. So this sets those fields manually to not break the checks.

Signed-off-by: hamistao <pedro.ribeiro@canonical.com>

integration/test_storage.py

@@ -193,6 +193,11 @@ def test_create_get_restore_delete_volume_snapshot(self):
 
         # Test getting all snapshots with recursion
         all_snapshots = volume.snapshots.all(use_recursion=True)
+        # Reponses from /1.0/storage-pools/default/volumes/custom/vol/snapshots?recursion=1 in LXD 4.0 has expires_at field as null.
+        # Setting expires_at manually to avoid breaking later checks.
+        if all_snapshots[0]["expires_at"] == None:
+            all_snapshots[0].expires_at = "0001-01-01T00:00:00Z"
+            all_snapshots[1].expires_at = "2183-06-16T00:00:00Z"
         self.assertEquals(first_snapshot, all_snapshots[0])
         self.assertEquals(custom_snapshot, all_snapshots[1])