Caching improvements related to bp_core_get_user_displayname()

- The cache check in BP_XProfile_Data::exists() should be aware that cache
  misses are stored as objects and check for misses accordingly
- Ensure that fullname cache is busted when field 1 is updated
- Some unit tests

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8022 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Improve cache test for BP_XProfile_ProfileData::exists()

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8023 cdf35c40-ae34-48e0-9cc9-0c9da1808c22