Fix memtest support when using dracut-live

No boot entry for memtest is created even when --memtest 'memtest86+' is passed to lb config if --initramfs dracut-live is also passed. This turns out to be because the memtest is being put in binary/boot for dracut-live, but it is put in binary/live when using initramfs-tools. This results in binary_grub_cfg being unable to find the memtest86+ executable, resulting in it being omitted from the boot menu.

Adding dracut-live to the list of initramfs systems that have the memtest86+ executable placed under binary/live resolves the issue.