Just a quick warning that neither those links have the information you need to on how to setup systemd-boot with the data from the package you installed.
I don’t have time to test this right now but the basic steps would be something like:
sudo mkdir /efi/EFI/memtest86
sudo cp /boot/memtest86+/memtest.efi /efi/EFI/memtest86/.
Then create a manual conf file in /efi/loader/entries
It should look something like this:
title memtest86
efi /EFI/memtest86/memtest.efi