Auto merge of #1585 - bitcartel:1583_stop_test_writing_to_testnet3, r=daira
Auto merge of #1585 - bitcartel:1583_stop_test_writing_to_testnet3, r=daira

Closes #1583 by setting up the datadir for the wallet gtest.

Stops test writing to ~/.zcash/testnet3.
Also related to #1506

bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash