Force startup-wrapper logic to run less times

[RobbieTheWagner]

Using { once: true } in the listener seems to fix our infinite number of calls. Fixes #1634

[DanMonroe]

@rwwagner90 I have a local version that both fixes the maximum call stack error AND the tests pass. Here is what I changed:

startup_wrapper.js:
change
window.EmberInspector = requireModule('ember-debug/main')['default'];

to

let emberDebugMainModule = requireModule('ember-debug/main');
if (!emberDebugMainModule['default']) {
  return;
}
window.EmberInspector = emberDebugMainModule['default'];