[5.3] Make afterApplicationCreated() callback registration public

[mfn]

Rational: when using e.g. PHPUnit Listener we receive the test class before a certain test has run.

This allows further inspection or adding additional stuff.

Unfortunately it doesn't allow registering additional after application callbacks from e.g. \PHPUnit_Framework_TestListener::startTest because the method \Illuminate\Foundation\Testing\TestCase::afterApplicationCreated isn't public => this PR changes this.

A concrete example what I'm doing: for every test in my Listener I'm registering a callback which in-depth analyzes the SQL statements performed. But I can only hook into DB::listen once the application is "started", as such I need to do this in the mentioned callbacks. But currently it's not directly possible to the visibility.

It's possible to workaround this but my understanding is that TestCase should be open in this regard to be more flexible, like in this scenario I described.