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/reliable_sender_test.go

@@ -71,7 +71,7 @@ func assertMsgsSent(t *testing.T, msgChan chan lnwire.Message,
 // a peer while taking into account its connection lifecycle works as expected.
 func TestReliableSenderFlow(t *testing.T) {
 	t.Parallel()
-	ctx := context.Background()
+	ctx := t.Context()
 
 	reliableSender := newTestReliableSender(t)
 
@@ -188,7 +188,7 @@ func TestReliableSenderFlow(t *testing.T) {
 // them as stale.
 func TestReliableSenderStaleMessages(t *testing.T) {
 	t.Parallel()
-	ctx := context.Background()
+	ctx := t.Context()
 
 	reliableSender := newTestReliableSender(t)