test: add rescan_utxos in MiniWallet's initialization

this simplifies usage when MiniWallet is used with a pre-mined chain.

test/functional/mempool_unbroadcast.py

@@ -23,7 +23,6 @@ class MempoolUnbroadcastTest(BitcoinTestFramework):
 
     def run_test(self):
         self.wallet = MiniWallet(self.nodes[0])
-        self.wallet.rescan_utxos()
         self.test_broadcast()
         self.test_txn_removal()