End-to-end tests for wrapped ERC20 transfers between accounts

[james-chf]

When a balance transfer is carried out, the EthBridge VP should only allow it if the sender of the transfer (i.e. owner of the balance which is being decreased) authorized the transaction.

This issue can be closed when we have tests for the following types of ERC20 transfers (similar to the generic multitoken transfer tests in #886):

• implicit -> implicit
• implicit -> established
• established -> implicit
• established -> established

[james-chf]

#8 is completed, so have widened the scope of this issue to cover wrapped ERC20 transfers for all accounts, not just established accounts. This issue depends on #578, after which we can specifically have tests for wrapped ERC20 transfers.