Write automated tests for growing system partition

The time spent on this is to be equally shared between Cyril and intrigeri (mentoring).

The new code that requires additional testing is https://git.tails.boum.org/tails/tree/config/chroot_local-includes/usr/share/initramfs-tools/scripts/init-premount/partitioning?h=feature/15292-repartition

New tests that might be worth writing:

• the FAT filesystem on the system partition is at least 4096M large
• the label of the FAT filesystem on the system partition is “Tails”
• the system partition is an ESP
• the FS UUID for the system partition is not a69020d2 (i.e. /dev/disk/by-uuid/A690-20D2 does not exist)
• the system partition has the expected GPT attributes/flags (regression test for #16178 (closed))

Expected GPT attributes:

SYSTEM_PARTITION_FLAGS = (
    1 << 0 |   # system partition
    1 << 2 |   # legacy BIOS bootable
    1 << 60 |  # read-only
    1 << 62 |  # hidden
    1 << 63    # do not automount
)

Feature Branch: test/16003-growing-system-partition

Parent Task: #16002 (closed)

Related issues

• Blocked by #15319 (closed)

Original created by @u on 16003 (Redmine)