Test artifacts

[ZoranPandovski]

Description

This adds a helper script for running the same unit test flow we use in CI. It:

1. installs MindsDB with the required handler extras and test dependencies
2. runs make unit_tests_slow
3. generates coverage outputs and HTML coverage reports
4. moves the produced artifacts into tests_artifacts/ for easier inspection

Type of change

(Please delete options that are not relevant)

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ⚡ New feature (non-breaking change which adds functionality)
• 📢 Breaking change (fix or feature that would cause existing functionality not to work as expected)
• 📄 This change requires a documentation update