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.
dee8ad3754