Adds unit tests for bp_core_time_since()

See #5017. See #5015

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7101 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Use _n() for plurals in bp_core_time_since()

The previous implementation used a hardcoded distinction between singular and
plurals, which is not flexible enough for many languages.

Fixes #5015

Props SergeyBiryukov

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7102 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
POT, generated from r7102

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7103 cdf35c40-ae34-48e0-9cc9-0c9da1808c22