[test] fundrawtransaction: lock watch-only shared address

self.nodes[0] creates an address which is watch-only-shared with self.nodes[3]. If nodes[0] spends the associated UTXO during any of its sends later, the watchonly test will fail, as nodes[3] now has insufficient funds.

Note that this also adds a new find_vout_for_address function to the test framework.

test/functional/test_framework/util.py

@@ -569,3 +569,14 @@ def mine_large_block(node, utxos=None):
     fee = 100 * node.getnetworkinfo()["relayfee"]
     create_lots_of_big_transactions(node, txouts, utxos, num, fee=fee)
     node.generate(1)
+
+def find_vout_for_address(node, txid, addr):
+    """
+    Locate the vout index of the given transaction sending to the
+    given address. Raises runtime error exception if not found.
+    """
+    tx = node.getrawtransaction(txid, True)
+    for i in range(len(tx["vout"])):
+        if any([addr == a for a in tx["vout"][i]["scriptPubKey"]["addresses"]]):
+            return i
+    raise RuntimeError("Vout not found for address: txid=%s, addr=%s" % (txid, addr))