Write tests for synchronizer_cron_child_sites()

[marcoarthur]

Writing tests for the main function of the sync plugin. That do the heavy job for requesting the child. Maybe we should break it down in smaller functions, so we can unit test them better.

[marcoarthur]

Class Child gets the following interface to handle Child sites:
Child( url ) constructor connects and retrieve posts from URL endpoint (/posts).
->sync() does the duty to make the mapping, saving unsaved or updated posts from URL

[marcoarthur]

Something need to be checked since only 7.2 is passing the test on travis. Still we can review it.
https://travis-ci.org/perlcoder/wp-integration/jobs/566174696#L610 (pass)
https://travis-ci.org/perlcoder/wp-integration/jobs/566174697#L659 (failed)
https://travis-ci.org/perlcoder/wp-integration/jobs/566174698#L661 (failed)