Fix all tests

- Dedup `perform_test_from_obj` and the test cases used in both
`test_floodsub` and `test_gossipsub_backward_compatibility.py`.
Therefore, they are put in the standalone file
`tests/pubsub/floodsub_integration_test_settings.py`. The functions
and testcases are imported from there then.
- IMO still need a refactor on the tests. There are still some duplicate
code.

libp2p/pubsub/gossipsub.py

@@ -32,8 +32,8 @@ class GossipSub(IPubsubRouter):
 
         # Store target degree, upper degree bound, and lower degree bound
         self.degree = degree
-        self.degree_high = degree_high
         self.degree_low = degree_low
+        self.degree_high = degree_high
 
         # Store time to live (for topics in fanout)
         self.time_to_live = time_to_live