Add a testcase mixin for avatar services

Review Request #8264 — Created June 30, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
4649cf7...

Reviewers

All unit tests that interact with the avatar service registry now
inherit from the AvatarServicesTestMixin, which ensures that the
registry and global SiteConfiguration objects are reset after each unit
test.

Ran unit tests.