test: switch wallet_crosschain.py to signet

Testnet3 is deprecated. Instead of moving to testnet4, which might
also be deprecated in the future, use signet.

Author: Sjors

test/functional/wallet_crosschain.py

@@ -20,10 +20,11 @@ def skip_test_if_missing_module(self):
     def setup_network(self):
         self.add_nodes(self.num_nodes)
 
-        # Switch node 1 to testnet before starting it.
-        self.nodes[1].chain = 'testnet3'
-        self.nodes[1].extra_args = ['-maxconnections=0', '-prune=550'] # disable testnet sync
-        self.nodes[1].replace_in_config([('regtest=', 'testnet='), ('[regtest]', '[test]')])
+        # Switch node 1 to any network different from regtest before starting it.
+        self.nodes[1].chain = 'signet'
+        # Disable network sync and prevent disk space warning on low resource CI
+        self.nodes[1].extra_args = ['-maxconnections=0', '-prune=550']
+        self.nodes[1].replace_in_config([('regtest=', 'signet='), ('[regtest]', '[signet]')])
         self.start_nodes()
 
     def run_test(self):