Xcode: Fix the generation of code coverage files

__gcov_flush() must be called from within the main app bundle, not from
the unit tests bundle to properly generate the .gcda files.
(see for reference: http://stackoverflow.com/a/19386835/1380781)

Also, it is very bad form to add .m files from your app to your tests 
target, because of how unit tests are injected into the main app bundle.

http://www.objc.io/issue-1/testing-view-controllers.html
> Again: Injection is complex. Your take away should be: Don’t add .m 
> files from your app to your test target. You’ll get unexpected behavior.