Use wallet balance before opening the channel for test assertions

[luckysori]

        "Balance after closing channel should equal balance after opening plus PTLC amount, plus fund amount, minus transaction fees"

I found this assertion unintuitive. I think it is safer (in the regards of not loosing money) to that the diff of balance before opening the channel and after closing the channel matches the payments done over the channel.

For example. if twice the fund_amount is taken from the balance to open the channel, then this assert would not catch the bug and ultimately the user would lose money.

Originally posted by @D4nte in #50 (comment)