1. Make TestServerGeneral inherit from TestServer because
we need to use servers_mock, image_mock and flavor_mock
in compute_client.
2. Create a dict containing all info of a server in the
original format, and pass it to _prep_server_detail().
3. Compare the original and formatted dict.
Change-Id: Ie0b83c42a5c3bbba630a064d28374d07e2ce9caf
Implements: blueprint osc-unit-test-framework-improvement