The execute() method has been deprecated for a while, but we didn't

actually do anything about that since the warning was hidden behind an

environment variable.
This change updates the rbtools.clients.tests.test_scanning module to

use run_process() instead, and tweaks our BaseSCMClientTestCase

definition to allow us to define test cases that use [None] as the

client (since this test case needs some of the machinery from that but

doesn't instantiate a specific client class).

Ran unit tests.