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.

autopilot/top_centrality_test.go

@@ -1,7 +1,6 @@
 package autopilot
 
 import (
-	"context"
 	"testing"
 
 	"github.com/btcsuite/btcd/btcec/v2"
@@ -59,7 +58,7 @@ func testTopCentrality(t *testing.T, graph testGraph,
 		// Attempt to get centrality scores and expect
 		// that the result equals with the expected set.
 		scores, err := topCentrality.NodeScores(
-			context.Background(), graph, channels, chanSize, nodes,
+			t.Context(), graph, channels, chanSize, nodes,
 		)
 
 		require.NoError(t, err)