(PE-5677) Add tests for gem cli tool.

Signed-off-by: Wayne <wayne@puppetlabs.com>
@waynr  you may want to add a helper that returns the state of the gems to the initial_installed_gems using a mechanism not under test. If you do so, it should be run in a teardown step in order to ensure that the state of the SUT has been returned to its initial condition. If any of the steps above fail, your SUT may have gems you don't want installed on it when the next test is run.