[trello.com/c/dmJg0inY]: DogeWalletService tests

[ChrisBenua]

Initially open PR to EthWalletService tests branch to minimise diff.

Used only two hacks:

1. DogeWallet raw initializer (marked it as deprecated with message "For testing purposes only" and made it available only in DEBUG builds)
2. DogeWalletService.setWalletForTests - method to set wallet easily for testing purposes, (marked it as deprecated with message "For testing purposes only" and made it available only in DEBUG builds)

In tests I caught every custom error thrown by createTransaction method, verified that parameters passed to BitcoinKit.Transaction.createTransactions are expected.

PR is very similar to BtcWalletServiceTest but has some unique caveats to workaround:

1. To use mocks for DogeApiService protocol DogeApiServiceProtocol was introduced, but I had to slightly change internals of DogeApiService to match protocol requirements (public interface didn't change)
2. DogeApiServiceProtocolMock redirect all calls to _api property, also api property (introduced to match protocol requirement) redirects all calls to _api, so whenever DogeWalletService calls network request using either dogeApiService.request or dogeApiService.api.request - we don't need extra mocks for each case