Make fixtures part of tests

By renaming the fixtures to fixtures_test, we mark them as test, and no longer part of the public API.

Some files were left out as they were included in other tests:

    $ grep "/testing" -R openstack | cut -d " " -f 2 | sort | uniq
    "github.com/gophercloud/gophercloud/openstack/common/extensions/testing"
    "github.com/gophercloud/gophercloud/openstack/identity/v3/tokens/testing"
    "github.com/gophercloud/gophercloud/openstack/networking/v2/extensions/layer3/floatingips/testing"
    "github.com/gophercloud/gophercloud/openstack/networking/v2/networks/testing"
    "github.com/gophercloud/gophercloud/openstack/networking/v2/ports/testing"
    "github.com/gophercloud/gophercloud/openstack/objectstorage/v1/accounts/testing"
    "github.com/gophercloud/gophercloud/openstack/objectstorage/v1/containers/testing"

This should prevent go-apidiff from complaining when modifying fixtures.

This commit mirrors 7f1d075 that merged
in master.

Author: mandre