refactor: move repeated mock in tests to its own mock file

[josefinalliende]

Description

This tiny PR moves a mock class for active pubkey provider that was repeated in 4 test files. I realized when I needed to mock this for 5th time when adding tests to #728

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore
• 🧪 Tests

Checklist

• Run just precommit to ensure that formatting and linting are correct
• Run just check-flutter-coverage to ensure that flutter coverage rules are passing
• Updated the CHANGELOG.md file with your changes (if they affect the user experience)

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch refactor/repeated-active-pubkey-provider-mock

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