REST API: don't check if a post is freshly pressed if `is_post_freshly_pressed()` is not defined (e.g. on a Jetpack site)

* on a Jetpack site, where we don't mark posts as freshly pressed anyway, `is_post_freshly_pressed()` is not defined, so in the REST API, we shouldn't execute any checks with it if it's not defined.
* adds tests to check that this functionality continues to work as expected in WP.com and that the fatal is fixed in Jetpack
* simplifies/cleans up the Jetpack and Core Post tests

see https://github.com/Automattic/jetpack/issues/1245

Merges r108054-wpcom.