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.

lnwallet/channel_test.go

@@ -3836,7 +3836,7 @@ func TestQuitDuringSignNextCommitment(t *testing.T) {
 	)
 
 	quitDelay := time.Millisecond * 20
-	quit, quitFunc := context.WithCancel(context.Background())
+	quit, quitFunc := context.WithCancel(t.Context())
 
 	// Alice's channel will be stuck waiting for aux sig job responses until
 	// we send the quit signal. We add an explicit sleep here so that we can