input/script_utils_test: refactor TestHTLCSenderSpendValidation
input/script_utils_test: refactor TestHTLCSenderSpendValidation

To be able to change more than the witness used for each test case, we
extract commit and sweep tx generation into own methods that can be
called from each test case.

We do the same for TestHTLCReceiverSpendValidation

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd