Update Base.tarball test so it don't rely on network

The `.tarball` test constantly fails on TravisCI and is overall pretty long to run locally.

I mocked the `download` NPM module that it façade to test it without sending request through the networks. This way, it'll stop failing Travis due to connections timeouts.

If we trust `download` module, I don't think it is necessary to test it on the network on our side.