Use _n() for translations in bp_get_group_member_count()

The previous technique (manually switching between 1 and not-1) does not
provide adequate internationalizability for all languages.

Adds unit tests.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9047 cdf35c40-ae34-48e0-9cc9-0c9da1808c22