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.
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