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.

msgmux/msg_router_test.go

@@ -34,7 +34,7 @@ func (m *mockEndpoint) SendMessage(ctx context.Context, msg PeerMsg) bool {
 // TestMessageRouterOperation tests the basic operation of the message router:
 // add new endpoints, route to them, remove, them, etc.
 func TestMessageRouterOperation(t *testing.T) {
-	ctx := context.Background()
+	ctx := t.Context()
 	msgRouter := NewMultiMsgRouter()
 	msgRouter.Start(ctx)
 	defer msgRouter.Stop()