Unit tests involving at mentions notifications fails when an activity is updated

[imath]

Tests suite in WP < 4.0 does not include the WP_UnitTestCase->_restore_hooks() function.
When updating an activity, the filter bp_activity_at_name_do_notifications is fired to prevent sending more than one notification. Once we've reached this filter all at_mentions tests are failing so we need to temporarly remove it during these tests

[imath]

In 9368:

Fix at mention notifications unit tests for WordPress < 4.0

Fixes #6129