Updates groupmeta unit tests to work with new WP API functions as well

The WP API has some additional caching requirements, so to make the tests work
for both the old and new functions, a few additional cache-clearings are
necessary.

See #4551

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7834 cdf35c40-ae34-48e0-9cc9-0c9da1808c22