Backend fIlter testing could be more comprehensive

[Henry Nash]

The current filter testing for backends covers some of the filtering combinations (such as startswith) . but not all of them. These should be expanded to provide better coverage (especially as filtering is now supported by SQL and Ldap backends).

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/293159

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/293159
Committed: https://git.openstack.org/cgit/openstack/keystone/commit/?id=1c0c7fcf91fd9a292f0cf09300d0673840e7fc54
Submitter: Jenkins
Branch: master

commit 1c0c7fcf91fd9a292f0cf09300d0673840e7fc54
Author: Colleen Murphy <email address hidden>
Date: Tue Mar 15 15:18:52 2016 -0700

    Make backend filter testing more comprehensive

    This patch adds additional checks to the inexact filter tests. It
    mostly cargo-cults the `test_list_users_inexact_filtered` test. In
    order to be consistent with the `test_list_users_inexact_filtered`
    test, it modifies the helper functions `_groups_for_user_data()` and
    `_list_users_in_group_data()` to not initialize the hints object, since
    the hints object now needs to be re-initialized between every filter
    type.

    Change-Id: I88b26406fcd25e30ea2beb7953c604576da38de3
    Closes-bug: #1424698