IUK creation: make saveas method benchmarking info honor its $outfile_name argument

This fixes t/specs/Create.t, which was broken by
df8e1f35: the `saveas` method shall not rely on
$self->outfile, but instead honor its $outfile_name argument, which matters for
unit testing. The `save` method, which is used by tails-create-iuk, will forward
the requested $self->outfile to the `saveas` method.

Similarly, the create_squashfs_diff method shall not rely on $self->outfile
being set.