Move the panel mocks to a separate method on TestCase

We need to remove the panel mocks in Tuskar-UI tests, because they
import and register the admin panel as their side effect. To do that
easily, I'm moving that part into a separate method.

Partial-bug: #1384683
Change-Id: Ica366c0b9f680049a74e94007a563ffdb492e4f4
Merge "Move the panel mocks to a separate method on TestCase"