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/update-expression/output.js

@@ -1,4 +1,2 @@
-function _readOnlyError(name) { throw new TypeError("\"" + name + "\" is read-only"); }
-
 var foo = 1;
-_readOnlyError("foo"), foo++;
+babelHelpers.readOnlyError("foo"), foo++;