Improving test_download_packages test

[itayporezky]

(FYI, Junior) I would like to attempt to contribute to this,

Would it be a smart idea to add checksum verification to: (https://github.com/pdm-project/pdm-download/blob/495cfd20a4533835d03772dc15da5b21f7e57686/tests/test_download.py#L32) of downloaded files vs the checksum pdm.static.lock found at file?

If so, would this be a seperate test or should i extend the existing test?