Maybe at the beginning of this line, we could say "You can create your own test helpers when you find yourself repeating setup behaviour while writing your tests. "

Also, we should change "the latter will not run until any previous async helper has"... to "until all previous async helpers have"... (ie change any helper to all helpers).
Also, we should probably end the sentence at "completed".

So, in other words, rewrite to the following (note I've replaced the last section with some different words that mean the same thing):

"`Ember.Test.registerHelper` and `Ember.Test.registerAsyncHelper` are used to register test helpers that will be injected when `App.injectTestHelpers` is called. The difference between `Ember.Test.registerHelper` and `Ember.Test.registerAsyncHelper` is that the latter will not run until all previous async helpers have completed. This allows you to form chains of sequenced helper calls out of code that would otherwise run in unsequenced order."