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.

lnrpc/routerrpc/router_backend_test.go

@@ -2,7 +2,6 @@ package routerrpc
 
 import (
 	"bytes"
-	"context"
 	"encoding/hex"
 	"testing"
 	"time"
@@ -264,7 +263,7 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
 		backend.MaxTotalTimelock = 1000
 	}
 
-	resp, err := backend.QueryRoutes(context.Background(), request)
+	resp, err := backend.QueryRoutes(t.Context(), request)
 
 	// If we're using both OutgoingChanId and OutgoingChanIds, we should get
 	// an error.

lnrpc/routerrpc/router_server_test.go

@@ -82,7 +82,7 @@ func TestTrackPaymentsReturnsOnCancelContext(t *testing.T) {
 	}
 	towerMock := makeControlTowerMock()
 
-	streamCtx, cancelStream := context.WithCancel(context.Background())
+	streamCtx, cancelStream := context.WithCancel(t.Context())
 	stream := makeStreamMock(streamCtx)
 
 	server := &Server{
@@ -110,7 +110,7 @@ func TestTrackPaymentsInflightUpdates(t *testing.T) {
 	}
 	towerMock := makeControlTowerMock()
 
-	streamCtx, cancelStream := context.WithCancel(context.Background())
+	streamCtx, cancelStream := context.WithCancel(t.Context())
 	stream := makeStreamMock(streamCtx)
 	defer cancelStream()
 
@@ -172,7 +172,7 @@ func TestTrackPaymentsNoInflightUpdates(t *testing.T) {
 	}
 	towerMock.queue.Start()
 
-	streamCtx, cancelStream := context.WithCancel(context.Background())
+	streamCtx, cancelStream := context.WithCancel(t.Context())
 	stream := makeStreamMock(streamCtx)
 	defer cancelStream()