Clean up tarball in old-cli-test.

If we don't clean up the tarball, then the following command (`meteor
build`) fails ("Couldn't create tarball"), but for some reason still
exits with code 0. So we then go on to successfully untar the output of
`meteor build`. If we clean up the tarball from `meteor bundle`, we at
least catch the failure by trying to untar a nonexistent file.