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.

invoices/kv_sql_migration_test.go

@@ -1,7 +1,6 @@
 package invoices_test
 
 import (
-	"context"
 	"database/sql"
 	"os"
 	"path"
@@ -67,7 +66,7 @@ func TestMigrationWithChannelDB(t *testing.T) {
 		sqlite bool) {
 
 		sqlInvoiceStore, sqlStore := makeSQLDB(t, sqlite)
-		ctxb := context.Background()
+		ctxb := t.Context()
 
 		const batchSize = 11
 		err := sqlStore.ExecTx(
@@ -163,7 +162,7 @@ func TestMigrationWithChannelDB(t *testing.T) {
 
 				sqliteBackend, err := kvdb.Open(
 					kvdb.SqliteBackendName,
-					context.Background(),
+					t.Context(),
 					sqliteConfig, test.dbPath,
 					lncfg.SqliteChannelDBName,
 					lncfg.NSChannelDB,