multi: context.Background() -> t.Context()

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.

discovery/syncer_atomic_test.go

@@ -15,7 +15,7 @@ import (
 // backlog at a time using the atomic flag.
 func TestGossipSyncerSingleBacklogSend(t *testing.T) {
 	t.Parallel()
-	ctx := context.Background()
+	ctx := t.Context()
 
 	// Track how many goroutines are actively sending.
 	var (