refactor load migrations

pkg/spanner/migration.go

@@ -85,10 +85,13 @@ func LoadMigrations(dir string) (Migrations, error) {
 		return nil, err
 	}
 
+	return LoadMigrationFiles(dir, files)
+}
+
+func LoadMigrationFiles(baseDir string, files []os.DirEntry) (Migrations, error) {
 	var migrations Migrations
 
 	versions := map[uint64]string{}
-
 	for _, f := range files {
 		if f.IsDir() {
 			continue
@@ -106,7 +109,7 @@ func LoadMigrations(dir string) (Migrations, error) {
 			continue
 		}
 
-		file, err := os.ReadFile(filepath.Join(dir, filename))
+		file, err := os.ReadFile(filepath.Join(baseDir, filename))
 		if err != nil {
 			continue
 		}