A testcase is missing for a full settlement to a partially settled IOU

[ooharawork]

In excercise of IOUContract.kt, I had erroneously ignored paid amount on settlement contract, i.e.

// wrong
val amountOutstanding = inputIou.amount
// correct
val amountOutstanding = inputIou.amount  - inputIou.paid

IOUSettleTests.kt couldn't detect the error. We needed a test case where we will fully settle an IOU that was partially settled, e.g.

1. Issue an IOU $10 -> output: one IOU (amount=10, paid=0)
2. Then settle $5 -> output: one IOU (amount=10, paid=5)
3. Then settle $5 -> no IOU output