trivial: Do not use g_file_load_bytes() in the self tests
trivial: Do not use g_file_load_bytes() in the self tests

This is not available in older GLib versions.

fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd