Refactor test and benchmark envs to use public managers

[pzuraq]

We have begun the process of upstreaming public managers from Ember.js so they can be shared across Glimmer and Ember, and so they can be used for strict mode. Currently, the test and benchmark env still use the low level manager infrastructure. We should continue refactors until they no longer do, and instead mirror the same infrastructure that is used in Ember.js and Glimmer.js.