AST-1579 Mock downloading in download test
Since tgssadr.strw.leidenuniv.nl does not always respond, mock it in the download_skymodel test so the tests no longer depend on this external server.
Edited by Maik Nijhuis
Since tgssadr.strw.leidenuniv.nl does not always respond, mock it in the download_skymodel test so the tests no longer depend on this external server.