Imitate other specs

spec/lib/backup/manager_spec.rb

@@ -195,7 +195,7 @@ describe Backup::Manager do
       end
 
       it 'prints the list of available backups' do
-        subject.unpack
+        expect { subject.unpack }.to raise_error SystemExit
         expect(progress).to have_received(:puts)
           .with(a_string_matching('1451606400_2016_01_01_1.2.3'))
       end