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_misc_test.go

@@ -1400,7 +1400,7 @@ func testGRPCNotFound(ht *lntest.HarnessTest) {
 // during a reorg. A reorg notification is produced after a reorg affects the
 // block which has produced a spending notification for this registration.
 func testReorgNotifications(ht *lntest.HarnessTest) {
-	ctxb := context.Background()
+	ctxb := ht.Context()
 	const timeout = wait.DefaultTimeout
 
 	alice := ht.NewNodeWithCoins("Alice", nil)
@@ -1599,7 +1599,7 @@ func testReorgNotifications(ht *lntest.HarnessTest) {
 func testEstimateFee(ht *lntest.HarnessTest) {
 	alice := ht.NewNode("Alice", nil)
 
-	ctx := context.Background()
+	ctx := ht.Context()
 
 	testCases := []struct {
 		name        string