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). Testing Done: Ran unit tests.