This commit introduces the Signature interface which will be used by our
witness construction methods instead of passing in raw byte slices. This
will be used later to inject various kinds of mock signatures, e.g.
73-byte signatures for simulating worst-case witness weight.
We also increase the witness size for these types to account for the 3
extra bytes. The size won't be correct in all cases, but it is just an
upper bound in any case.
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
This fixes an error case that wouldn't have been caught, since
vm.Execute applies more rules than the individual steps (most notably
the clean stack rule).
Instead we execute the engine as normal, and only step through if we
decide that the outcome is unexpected.
This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
