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

Signed-off-by: Wayne <wayne@puppetlabs.com>
@waynr will this remove gems that are needed for subsequent tests?

Suggestion: Only uninstall gems that are not in `initial_installed_gems` list.

    gems.each do |gem_name|
        on(master, "#{gem_uninstall} #{gem_name}") unless initial_installed_gems.include?(gem_name)
    end