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/multiple/output.js

@@ -1,6 +1,4 @@
-function _skipFirstGeneratorNext(fn) { return function () { var it = fn.apply(this, arguments); it.next(); return it; }; }
-
-_skipFirstGeneratorNext(function* () {
+babelHelpers.skipFirstGeneratorNext(function* () {
   let _functionSent = yield;
 
   const a = _functionSent;