We now have both `Amount::from_sat_unchecked` and
`SignedAmount::from_sat_unchecked`. These constructors are explicitly
for ignoring any invariant (implied or otherwise) especially in test
code.
Note we do not enforce an invariant currently. This patch is a baby step
towards getting the `amount` module in order.
Replace all calls to `from_sat` for const int values with the
`_unchecked` constructor. Done in `amount::tests` only.
04bae4bb91