test: add rescan_utxos in MiniWallet's initialization

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

test/functional/p2p_blocksonly.py

@@ -20,8 +20,6 @@ class P2PBlocksOnly(BitcoinTestFramework):
 
     def run_test(self):
         self.miniwallet = MiniWallet(self.nodes[0])
-        # Add enough mature utxos to the wallet, so that all txs spend confirmed coins
-        self.miniwallet.rescan_utxos()
 
         self.blocksonly_mode_tests()
         self.blocks_relay_conn_tests()