[wallet] randomly generate change targets

If the wallet always chooses 1 million sats as its change target, it is
easier to fingerprint transactions created by the Core wallet.

test/functional/wallet_bumpfee.py

@@ -442,7 +442,9 @@ def test_watchonly_psbt(self, peer_node, rbf_node, dest_address):
     self.generate(peer_node, 1)
 
     # Create single-input PSBT for transaction to be bumped
-    psbt = watcher.walletcreatefundedpsbt([], {dest_address: 0.0005}, 0, {"fee_rate": 1}, True)['psbt']
+    # Ensure the payment amount + change can be fully funded using one of the 0.001BTC inputs.
+    psbt = watcher.walletcreatefundedpsbt([watcher.listunspent()[0]], {dest_address: 0.0005}, 0,
+            {"fee_rate": 1, "add_inputs": False}, True)['psbt']
     psbt_signed = signer.walletprocesspsbt(psbt=psbt, sign=True, sighashtype="ALL", bip32derivs=True)
     psbt_final = watcher.finalizepsbt(psbt_signed["psbt"])
     original_txid = watcher.sendrawtransaction(psbt_final["hex"])