Enable external-helpers by default in tests (#12911)

Co-authored-by: Huáng Jùnliàng <jlhwung@gmail.com>
Co-authored-by: Babel Bot <babel-bot@users.noreply.github.com>

packages/babel-plugin-transform-block-scoping/test/fixtures/const-violations/loop/output.js

@@ -1,5 +1,3 @@
-function _readOnlyError(name) { throw new TypeError("\"" + name + "\" is read-only"); }
-
-for (var i = 0; i < 3; i = (_readOnlyError("i"), i + 1)) {
+for (var i = 0; i < 3; i = (babelHelpers.readOnlyError("i"), i + 1)) {
   console.log(i);
 }