net: Give seednodes time before falling back to fixed seeds

Before, we'd remove a seednode from the list right after connecting
to it, leading to a race with loading the fixed seed and connecting
to them.

test/functional/feature_config_args.py

@@ -213,7 +213,7 @@ class ConfArgsTest(BitcoinTestFramework):
         with self.nodes[0].assert_debug_log(expected_msgs=[
                 "Loaded 0 addresses from peers.dat",
                 "DNS seeding disabled",
-                "Adding fixed seeds as -dnsseed=0 (or IPv4/IPv6 connections are disabled via -onlynet), -addnode is not provided and all -seednode(s) attempted\n",
+                "Adding fixed seeds as -dnsseed=0 (or IPv4/IPv6 connections are disabled via -onlynet) and neither -addnode nor -seednode are provided\n",
         ]):
             self.start_node(0, extra_args=['-dnsseed=0', '-fixedseeds=1'])
         assert time.time() - start < 60