[CBR-465] Sort payees by amount in coin selection tests
[CBR-465] Sort payees by amount in coin selection tests

One of our tests picked up a corner case in the generators and in the
largestFirst policy, where big UTxO entries were going to be picked to
satisfy small payments, leaving beefy ones "uncovered". The fix here was
simply to sort the payees by amount _in the tests_, not in the coin
selection code. The reason is simple:

* We don't use @largestFirst@ in production code. We rely only on the
  @atLeast@ low-level API inside the @random@ policy, which is the one
  we use in production;
* We have tests hitting @largestFirst@ only for the sake of implicitly
  testing @atLeast@;
* The @random@ policy doesn't require this sorting precondition.

Therefore, embedding this sorting into the main coin selection code
would have been a waste of CPU cycles. Doing it in the tests only
suffices.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl