test: replace ValidWitnessMalleatedTx class with function

Simplify the witness malleation test helper by converting the
ValidWitnessMalleatedTx class to a standalone function
build_malleated_tx_package() and updating call sites.

Co-authored-by: rkrux <rkrux.connect@gmail.com>

test/functional/mempool_accept_wtxid.py

@@ -7,17 +7,16 @@ Test mempool acceptance in case of an already known transaction
 with identical non-witness data but different witness.
 """
 
-from test_framework.messages import (
-    COIN,
-)
 from test_framework.p2p import P2PTxInvStore
-from test_framework.script_util import ValidWitnessMalleatedTx
+from test_framework.script_util import build_malleated_tx_package
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_not_equal,
     assert_equal,
 )
-
+from test_framework.wallet import (
+    MiniWallet,
+)
 
 class MempoolWtxidTest(BitcoinTestFramework):
     def set_test_params(self):
@@ -25,23 +24,27 @@ class MempoolWtxidTest(BitcoinTestFramework):
 
     def run_test(self):
         node = self.nodes[0]
+        mini_wallet = MiniWallet(node)
         self.log.info('Start with pre-generated blocks')
-        blockhash = self.nodes[0].getblockhash(1)
-        txid = node.getblock(blockhash=blockhash, verbosity=2)["tx"][0]["txid"]
+
         assert_equal(node.getmempoolinfo()['size'], 0)
 
         self.log.info("Submit parent with multiple script branches to mempool")
-        txgen = ValidWitnessMalleatedTx()
-        parent = txgen.build_parent_tx(txid, 9.99998 * COIN)
 
-        privkeys = [node.get_deterministic_priv_key().key]
-        raw_parent = node.signrawtransactionwithkey(hexstring=parent.serialize().hex(), privkeys=privkeys)['hex']
-        signed_parent_txid = node.sendrawtransaction(hexstring=raw_parent, maxfeerate=0)
+        parent = mini_wallet.create_self_transfer()["tx"]
+        parent_amount = parent.vout[0].nValue - 10000
+        child_amount = parent_amount - 10000
+        parent, child_one, child_two = build_malleated_tx_package(
+            parent=parent,
+            rebalance_parent_output_amount=parent_amount,
+            child_amount=child_amount
+        )
+
+        mini_wallet.sendrawtransaction(from_node=node, tx_hex=parent.serialize().hex())
+
         self.generate(node, 1)
 
         peer_wtxid_relay = node.add_p2p_connection(P2PTxInvStore())
-
-        child_one, child_two = txgen.build_malleated_children(signed_parent_txid, 9.99996 * COIN)
         child_one_wtxid = child_one.wtxid_hex
         child_one_txid = child_one.txid_hex
         child_two_wtxid = child_two.wtxid_hex