A utility class that you can derive from in your unit tests. TestWithWallet sets up a wallet with a key,
an in memory block store and a block chain object. It also provides helper methods for filling the wallet
with money in whatever ways you wish. Note that for simplicity with amounts, this class sets the default
fee per kilobyte to zero in setUp and back to normal in tearDown. If you are wanting to test your behaviour
with fees (a good idea!) make sure you set the Wallet.SendRequest#DEFAULT_FEE_PER_KB value to
Transaction.REFERENCE_DEFAULT_MIN_TX_FEE before doing so.