Split up activity `'date_query'` unit tests.

Attempts to fix issues with the 'zend_mm_heap corrupted' message outputting
on PHP 5.2 and WP master:
https://travis-ci.org/buddypress/BuddyPress/jobs/32239701

See #5803.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8789 cdf35c40-ae34-48e0-9cc9-0c9da1808c22