Add expiry integration tests

[marsella]

Add the following happy tests:

• [ ] close on a channel with all balance owed the customer
• [ ] close on a channel with all balance owed the merchant
• [ ] close on a channel with balance for each party

and the following tests, which should error:

• [ ] close on a channel after a store/pay/restore series. All funds should go to the merchant.
• [ ] close on a channel that was not established. Active op should error.

...and any others you think of.

In all of these, "close" is, of course, an expiry-initialized close.