pytest: Skip test_sign_and_send_psbt for elements

This test is flaky because the generated PSBT seems to not have the change
output adjusted, or it is missing.

Tracking-Issue: ElementsProject/lightning#3998

tests/test_wallet.py

@@ -618,6 +618,8 @@ def test_utxopsbt(node_factory, bitcoind):
                     reservedok=True)
 
 
+@unittest.skipIf(TEST_NETWORK == 'liquid-regtest',
+                 "See ElementsProject/lightning#3998")
 def test_sign_and_send_psbt(node_factory, bitcoind, chainparams):
     """
     Tests for the sign + send psbt RPCs