Write more end to end tests

[rgayon]

• Write test for "failing" disks
• Write test with multiple attached disks

[rgayon]

Note to self:

Create a fake bad block device:

truncate -s 100M fake_partition; mkfs.ext4 fake_partition; losetup loop0 fake_partition
dmsetup create "baddisk" --table "0 10000 flakey /dev/loop0 3000 1 1"

cleanup:

dmsetup remove baddisk
losetup -d loop0