[qa] Sort scripts by time for pull_tester and don't overwrite setup_chain

qa/rpc-tests/p2p-mempool.py

@@ -72,8 +72,11 @@ class TestNode(NodeConnCB):
         self.send_message(msg_mempool())
 
 class P2PMempoolTests(BitcoinTestFramework):
-    def setup_chain(self):
-        initialize_chain_clean(self.options.tmpdir, 2)
+
+    def __init__(self):
+        super().__init__()
+        self.setup_clean_chain = True
+        self.num_nodes = 2
 
     def setup_network(self):
         # Start a node with maxuploadtarget of 200 MB (/24h)