memtest86plus: disable all hardening

With hardening enabled it reports errors on known-good memory modules
on my Thinkpad X230 (Ivy Bridge). It's the same bug as reported in
https://bugs.launchpad.net/ubuntu/+source/memtest86+/+bug/1071209 but
memtest86+ fails on test #9 instead of test #7 (because #7 in 4.20
became #9 in 5.01) and with all the addresses multiplied by 2 (I guess
the bug was reported for i686, and I test on x86_64, it was 2012 after
all).