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.

itest/lnd_route_blinding_test.go

@@ -335,7 +335,7 @@ type blindedForwardTest struct {
 func newBlindedForwardTest(ht *lntest.HarnessTest) (context.Context,
 	*blindedForwardTest) {
 
-	ctx, cancel := context.WithCancel(context.Background())
+	ctx, cancel := context.WithCancel(ht.Context())
 
 	return ctx, &blindedForwardTest{
 		ht:       ht,