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-proposal-function-sent/test/fixtures/function-sent/basic/output.js

@@ -1,11 +1,9 @@
-function _skipFirstGeneratorNext(fn) { return function () { var it = fn.apply(this, arguments); it.next(); return it; }; }
-
 function gen() {
   return _gen.apply(this, arguments);
 }
 
 function _gen() {
-  _gen = _skipFirstGeneratorNext(function* () {
+  _gen = babelHelpers.skipFirstGeneratorNext(function* () {
     let _functionSent = yield;
 
     let sent = _functionSent;