Fix unit test breakage for getYn validation

rbuild_test/unit_test/uitest.py:
  Add explicit test for getYn validation function

rbuild_test/unit_test/pluginstest/buildtest.py
  Mock the getYn function directly, since that's what warnIfOldProductDefinition calls

rbuild_test/unit_test/pluginstest/descriptorconfigtest.py:
  Add a return for 'more vhosts?' so that the validationFn won't keep
  getYn from prompting forever

rbuild_test/unit_test/pluginstest/configcommandtest.py:
  Add responses for getYn prompts