In test_get_with_display_comments_stream(), don't use bp_activity_new_comment()

The latter function does not accept a recorded_time argument - the current time
is assumed. As a result, on some systems (eg Travis) that run the tests fairly
slowly, the reverse chronological sort order of BP_Activity_Activity::get()
gets messed up, and the test fails. Calling bp_activity_add() directly allows
us to specify the correct recorded_time, fixing the test.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7155 cdf35c40-ae34-48e0-9cc9-0c9da1808c22