Exports: Exclude `wp_sync_storage` post type from exports.

Configured the Real Time Collaboration post type to be excluded from exports by default. The data is considered ephemeral and includes data on post IDs that may not match the IDs of posts on the importing site.

Introduces a test to the export test suite to ensure that post types set to be excluded from exports are, in fact, excluded from exports.

Props peterwilsoncc, desrosj, westonruter, jorbin, mukesh27, czarate.
Fixes #64964.




git-svn-id: https://develop.svn.wordpress.org/trunk@62168 602fd350-edb4-49c9-b593-d223f7449a82