Tests: stop manually resetting `$bp->avatar` between tests.

This causes tests to fail if those tests call `friends_add_friend()` inside `setUp()`
as the `$bp->avatar->show_avatars` property is unset. Instead, rather than manually
resetting the variable, we'll let `bp_core_set_avatar_globals()`, via `bp_setup_globals`
in `assertPreConditions()`, reset it correctly.

Fixes #5727

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8555 cdf35c40-ae34-48e0-9cc9-0c9da1808c22