The replacement testcases not being available is mostly my fault sorry. Since 18.10 we’ve used a testing-checklist where in the comments I provide machine details first line, second line is usually the testcase based from https://phab.lubuntu.me/w/release-team/testing-checklist/
testcase: full disk, no-encryption, BIOS, no-internet
where my comma separated values are the various options taken from prior checklist (ie. encryption/no-encryption, bios/uefi/secure, yes|no internet.
When I’m able, I’ll delete the last attempt to have testcases amended & start afresh (re-branch, re-add, commit etc). I’ve attempted to do that for some time, but haven’t got there yet (it didn’t look good when popey video did his video and they weren’t there;…)
Your issue with “sfdisk --force --append /dev/sda” looks like https://bugs.launchpad.net/ubuntu/+source/calamares/+bug/1864787 (where you’ll also note I link it as related to 1864791), so I’d encourage you to have a look at it, if it applies click “affects me too” & use that in iso.qa,ubuntu.com bug tracking for iso testing. I’ve only had that issue on a single box myself, and could not re-create it on another dell 755 optiplex box (ie. same make/model/look-box, but a different motherboard as model numbers can mean little).
You may also note in that bug report I was using BTRFS which I’d not use myself, but I was following a testcase found on the aforementioned checklist.