Write automated tests for growing system partition

Originally created by @Anonymous on #16003 (Redmine)

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)