Optionally enable external-helpers in tests (#12982)

* Optionally enable external-helpers in tests

#12911 was a breaking change that now requires you to keep `babel-plugin-external-helpers` in your `package.json`. It'll will probably be unnoticeable for most, because another dependency could transitively depend on it, but AMP recently hit it when cleaning up our deps.

* Cast to boolean

* Fix package name

packages/babel-helper-fixtures/src/index.ts

@@ -135,7 +135,9 @@ function pushTask(taskName, taskDir, suite, suiteName) {
         ? taskOpts.BABEL_8_BREAKING === false
         : taskOpts.BABEL_8_BREAKING === true),
     options: taskOpts,
-    externalHelpers: taskOpts.externalHelpers ?? true,
+    externalHelpers:
+      taskOpts.externalHelpers ??
+      !!tryResolve("@babel/plugin-external-helpers"),
     validateLogs: taskOpts.validateLogs,
     ignoreOutput: taskOpts.ignoreOutput,
     stdout: { loc: stdoutLoc, code: readFile(stdoutLoc) },