PM-12976: Fix test that needs main actor attribute

[matt-livefront]

🎟️ Tracking

PM-12976

📔 Objective

Fixes a test build error because of a missing @MainActor attribute for accessing the MockConfigService.

This is an addition to https://github.com/bitwarden/ios/pull/987 which was missed in https://github.com/bitwarden/ios/pull/988.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[github-actions[bot]]

Checkmarx One – Scan Summary & Details – f4cfcdca-b642-4f83-b43c-9b39bd4cf024

No New Or Fixed Issues Found

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.82%. Comparing base (60c6c57) to head (fe00dce). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #989 +/- ## ========================================== + Coverage 88.81% 88.82% +0.01% ========================================== Files 641 641 Lines 40215 40219 +4 ========================================== + Hits 35716 35725 +9 + Misses 4499 4494 -5 ```

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