Add mutual close tests

[marsella]

Add the following tests of mutual close

• [x] Mutual close on a channel where the customer has all the money works
• [x] Mutual close on a channel where the merchant has all the money works
• [x] Mutual close on a channel where both parties have some money works
• [ ] Mutual close after a store/pay/restore series of operations gives all the money to the merchant
• [x] Mutual close on a channel that has not been established, fails