Create and use a wallet.dat with miner address for QA tests

[oxarbitrage]

Before zallet can start in the QA tests, we need to ensure that a miner address is already present in its database.

One way to achieve this is by creating a wallet.dat file containing a transparent address, and then importing it into zallet using the functionality introduced in zcash/wallet#152.

blocked on zcash/wallet#152