test: ensure banlist can be read from disk after restart

With `banlist.dat` (being written in addrv1 format) if we would try to
write a Tor v3 subnet, it would serialize as a dummy-all-0s IPv6
address and subsequently, when deserialized will not result in the same
subnet.

This problem does not exist with `banlist.json` where the data is saved
in textual, human-readable form.

test/functional/rpc_setban.py

@@ -51,12 +51,24 @@ class SetBanTests(BitcoinTestFramework):
         ip_addr = "1.2.3.4"
         assert(not self.is_banned(node, tor_addr))
         assert(not self.is_banned(node, ip_addr))
+
         node.setban(tor_addr, "add")
         assert(self.is_banned(node, tor_addr))
         assert(not self.is_banned(node, ip_addr))
+
+        self.log.info("Test the ban list is preserved through restart")
+
+        self.restart_node(1)
+        assert(self.is_banned(node, tor_addr))
+        assert(not self.is_banned(node, ip_addr))
+
         node.setban(tor_addr, "remove")
         assert(not self.is_banned(self.nodes[1], tor_addr))
         assert(not self.is_banned(node, ip_addr))
 
+        self.restart_node(1)
+        assert(not self.is_banned(node, tor_addr))
+        assert(not self.is_banned(node, ip_addr))
+
 if __name__ == '__main__':
     SetBanTests().main()