Manual snapshot upgrade test

This MR aims to persist a tezt test that was used in the scope !14398 (merged) to test the consistency of snapshot imports.

It adds two test:

• one is importing a legacy snapshot (v7) that aims to be upgraded automatically
• one is importing a regular snapshot (v8)

This test is not expected to be generic enough to be generalized. As it may be used only once and not run on the CI, it is not necessary to look for a very high code quality.

This MR is a followup of !14401 (merged)

Manually testing the MR

NETWORK="<ntw>" SNAPSHOT_FILE="./v7_snapshot" SNAPSHOT_IMPORT_DIR="./import" dune exec tezt/manual_tests/main.exe -- --title "Check v.3.2 snapshot backward compatibility" --verbose

and

NETWORK="<ntw>" SNAPSHOT_FILE="./v8_snapshot" SNAPSHOT_IMPORT_DIR="./import" dune exec tezt/manual_tests/main.exe -- --title "Check v.3.2 new snapshot" --verbose

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR