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/deadcode-violation/output.js

@@ -1,7 +1,5 @@
-function _readOnlyError(name) { throw new TypeError("\"" + name + "\" is read-only"); }
-
 (function () {
   var a = "foo";
-  if (false) a = (_readOnlyError("a"), "false");
+  if (false) a = (babelHelpers.readOnlyError("a"), "false");
   return a;
 })();