Webpack guide is missing fake-async-test.js in testing configuration

[ericmartinezr]

Currently the configuration https://angular.io/docs/ts/latest/guide/webpack.html#!#test-configuration is missing this file that may cause issues. See this thread for more angular/angular#8232 and this comment specifically angular/angular#8232 (comment).

Probably this will be unnecessary if this is implemented angular/angular#8353 but it may be good too keep an eye on this issue.

cc : @Foxandxss

[Foxandxss]

Didn't know about that. Will check.