test: Restructure guest_disk tests

[frankdavid]

Move some of the shared logic from tests into the fixture so tests can be written with higher-level helper functions. The modified test fixture models the real GuestOS setup more closely.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 8293ae6.

Security Overview

• 🔎 Scanned files: 1 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/6992424c-495e-4621-b441-efd6821a4be6?scanId=f3c43758-39de-4617-be8a-a9beeb02edc0&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Introduce extensive tests for guest disk encryption tooling, including SEV-related flows, partition handling, and key management
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Add new TestFixture, PartitionView, BootSlot abstractions, and helper utilities for SEV-driven scenarios
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Add support for detached and attached LUKS headers, keyslot metadata, and SEV key derivation
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Expand test coverage to verify key exchange, header backup, and store/var partition behaviors under SEV and non-SEV configurations
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Introduce upgrade_sev_guestos_to and rollback flows to simulate SEV-based upgrades and downgrades
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Add tests for opening stores with previous keys, verifying metadata, and ensuring proper header usage
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Add tests for failing to open unformatted devices and detach header handling on format attempts
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Include utilities to decompose and inspect LUKS2 header locations and keyslots
Enhancement	► rs/ic_os/os_tools/guest_disk/src/tests.rs      Update and extend tests to validate SEV unlock sequences, detached header usage, and token backfills

[Copilot]

Pull request overview

Refactors the guest_disk test suite by moving repeated low-level setup logic into a richer test fixture that more closely models the real GuestOS disk topology (shared Store + A/B Var slots), enabling higher-level upgrade/rollback-style tests.

Changes:

• Introduces TestFixture with A/B BootSlots and a PartitionView helper to encapsulate per-partition operations (format/open/header inspection/mapper IO).
• Updates existing tests to use the higher-level helpers and adds slot-aware upgrade/rollback flows.
• Cleans up and centralizes common helpers (e.g., key derivation, keyslot counting, detached header handling).

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