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_graph_migration_test.go

@@ -21,7 +21,7 @@ func testGraphMigration(ht *lntest.HarnessTest) {
 		ht.Skip("not running with test_native_sql tag")
 	}
 
-	ctx := context.Background()
+	ctx := ht.Context()
 	alice := ht.NewNodeWithCoins("Alice", nil)
 
 	// Make sure we run the test with SQLite or Postgres.