Unit tests: make sure the user is a member of the group before promoting him

WordPress changeset 30400 introduced {{{STRICT_ALL_TABLES}}} in WPDB and revealed a trouble in {{{BP_Tests_Group_Cache->test_groups_get_group_admins_cache}}}. As the user was directly promoted as an admin of the group without being first added as a member, a WordPress database error was making this test fail.
The ticket will remain open in case we find some other notices errors when we use WPDB.

See #6036

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9156 cdf35c40-ae34-48e0-9cc9-0c9da1808c22