Implement E2E Test Suite for System Pallet (esp. runtime upgrades!)

[rockbmb]

Description

As the FRAME pallet source code says,

//! The System pallet provides dispatchable functions that, with the exception of `remark`, manage
//! low-level or privileged functionality of a Substrate-based runtime.

While other PET tests use system operations indirectly, there is no dedicated E2E test suite covering the critical runtime upgrade flows and system-level operations.

Objectives

Implement E2E tests covering key System pallet flows:

• Runtime Upgrade Mechanisms: Test set_code() for direct upgrades, authorize_upgrade() + apply_authorized_upgrade() for two-phase upgrades, and set_code_without_checks() for emergency scenarios
• Storage Management: Test set_storage(), kill_storage(), and kill_prefix() for administrative storage operations
• System Operations: Test the remark functions, set_heap_pages(), and do_task() for utility functions and system configuration
• Edge Cases: Test upgrade authorization conflicts, storage operation limits, multi-block migration interactions, and integration with governance for upgrade approvals

[karolk91]

as discussed with @rockbmb, let's assume this issue is assigned to me since we were unable to do the assignment on github (due to permissions)