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.

htlcswitch/switch_test.go

@@ -1,7 +1,6 @@
 package htlcswitch
 
 import (
-	"context"
 	"crypto/rand"
 	"crypto/sha256"
 	"errors"
@@ -3547,7 +3546,7 @@ func (n *threeHopNetwork) sendThreeHopPayment(t *testing.T) (*lnwire.UpdateAddHT
 	}
 
 	err = n.carolServer.registry.AddInvoice(
-		context.Background(), *invoice, htlc.PaymentHash,
+		t.Context(), *invoice, htlc.PaymentHash,
 	)
 	require.NoError(t, err, "unable to add invoice in carol registry")