Add Test Infrastructure for License Download Functionality

[bhorsujal]

Problem

Currently, there's no way to test critical functionality like license download failures, atomic updates, and error handling without manually breaking production systems or waiting for real-world failures.
Mainly to test the change implemented in #141

Proposed Solution

Implement a proper test infrastructure with:

1. License Download Testing
   Ability to simulate individual license download failures
   Test atomic update behavior when some downloads fail
   Verify progress reporting and error handling
2. UI Testing
   Test download status display in options page
   Verify failed download warnings and success messages
   Test permission error scenarios

Benefits

• Confidence in critical fault-tolerance features
• Easier debugging of edge cases
• Better development workflow for future license-related features
• Reduced risk when deploying updates

[bhorsujal]

@alandtse Please add enhancement and testing labels to this issue. Thank you.

[alandtse]

I'm pretty sure tags can be added by anyone.