fix: test optimizer to import files as valid dart names

[alestiago]

Description

Instead of importing files as a snaked case name of the relative path, the test files are assigned an alphabetic identifier.

Before:

import 'something[1]_test.dart' as something[1]_test; // Invalid dart identifier

After:

import 'something[1]_test.dart' as _a;

Resolves #648

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