test: add rescan_utxos in MiniWallet's initialization

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

test/functional/rpc_scanblocks.py

@@ -27,7 +27,6 @@ class ScanblocksTest(BitcoinTestFramework):
     def run_test(self):
         node = self.nodes[0]
         wallet = MiniWallet(node)
-        wallet.rescan_utxos()
 
         # send 1.0, mempool only
         _, spk_1, addr_1 = getnewdestination()