lnwallet: expand chan sync tests to cover taproot channels

In this commit, we expand some of the existing chan sync tests to cover
taproot channels (the others already did). Along the way, we always
assert that the `PartialSig` is populated on retransmission. In
addition, we now send the new commit sig rather than the existing
in-memory one to test the new logic that re-signs the commitment.

Author: Roasbeef