Use the new feature of Go 1.24, fix linter warnings.
This change was produced by:
- running golangci-lint run --fix
- sed 's/context.Background/t.Context/' -i `git grep -l context.Background | grep test.go`
- manually fixing broken tests
- itest, lntest: use ht.Context() where ht or hn is available
- in HarnessNode.Stop() we keep using context.Background(), because it is
called from a cleanup handler in which t.Context() is canceled already.
In this commit, we populate the resolution blobs for the incoming and
outgoing HTLCs. We take care to populate the AuxSigDesc with the correct
information, as we need to pass along the second-level aux signature and
also sign desc along with it.
