Since Go 1.22 loop variables are scoped per-iteration, so the
`x := x` / `a, b := a, b` copies inside range/for loops are no longer
needed. This removes the existing redundant copies in test files.
We'll use this AMP-specific ShardTracker for AMP payments. It will be
used to derive hashes for each HTLC attempt using the underlying AMP
derivation scheme.