The less you fake, the better the chances that your test is actually worth something: every time you stub something or fake it, you are asserting "I expect this code to perform the way I assume it will perform during setup", rather than "I expect this code to perform as it actually performs in the real world".

So, you, it would be much better to actually unpack something (and we have an ISO file for the purpose, even) than to fake it like this.