RPC tests: fixes merkle_blocks.py to exclude 1st coinbase

[marcus290]

Fixes one of the tests in #416.

merkle_blocks.py fails randomly on Travis (see https://travis-ci.org/NAVCoin/navcoin-core/jobs/493122877) due to "absurdly-high-fee" error here:

        node0utxos = self.nodes[0].listunspent(1)
        tx1 = self.nodes[0].createrawtransaction([node0utxos.pop()], {self.nodes[1].getnewaddress(): 49.99})
        txid1 = self.nodes[0].sendrawtransaction(self.nodes[0].signrawtransaction(tx1)["hex"])

node0utxos is a list of 55 unspent coinbase outputs from generating 55 blocks in the test environment, which are mostly 50 NAV each. However, the first coinbase is 59,800,000 NAV and if this is popped from the list, it will cause a fee error.

Setting maximum confirmations to listunspent() will avoid the 1st coinbase.