test: add rescan_utxos in MiniWallet's initialization

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

test/functional/mempool_spend_coinbase.py

@@ -28,7 +28,6 @@ class MempoolSpendCoinbaseTest(BitcoinTestFramework):
         chain_height = 198
         self.nodes[0].invalidateblock(self.nodes[0].getblockhash(chain_height + 1))
         assert_equal(chain_height, self.nodes[0].getblockcount())
-        wallet.rescan_utxos()
 
         # Coinbase at height chain_height-100+1 ok in mempool, should
         # get mined. Coinbase at height chain_height-100+2 is