tests: run `udevadm settle` after `sfdisk` (#8610)
This makes the script wait for the newly created partition to
show up before trying to put a filesystem on it, which should
prevent the tests from failing with the following error:
```
New situation:
Disklabel type: dos
Disk identifier: 0x3541a0ec
Device Boot Start End Sectors Size Id Type
/dev/loop6p1 2048 800767 798720 390M 83 Linux
/dev/loop6p2 800768 819199 18432 9M 83 Linux
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.
The file /dev/loop6p1 does not exist and no size was specified.
make: *** [setup] Error 1
F: Failed to mkfs -t ext4
Makefile:4: recipe for target 'setup' failed
```