Declare user_email when setting up fixtures for `count_user_posts()` tests.

Because `setUpBeforeClass()` is called statically and creates its own factory
object, the iterator logic from previous tests is not carried over to these
fixtures, and we risk the possibility of user_email clashes when the tests are
run in certain limited configurations.

git-svn-id: https://develop.svn.wordpress.org/trunk@30512 602fd350-edb4-49c9-b593-d223f7449a82