Tests: Change 'right now' condition test for `bp_core_time_since()` to '1 minute ago'

boonebgorges was right.  The 'right now' condition is too finicky to test
for.  Caused this unit test to fail in some instances.

See #4310.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8788 cdf35c40-ae34-48e0-9cc9-0c9da1808c22