Run praktika fast tests in GitHub Actions workflow

[tilman-aiven]

This PR adds optional CI testing for Aiven ClickHouse branches that never blocks merging. Tests run automatically on v*-lts-aiven* and v*-lts-upstream* branches and post results as PR comments.

Only runs when label clickhouse-fasttests is set

What Fast Test Does:

Builds ClickHouse (10-15 min)

• Fast build configuration
• No heavy optimizations
• Validates compilation

Runs ~10,000+ Tests (5-10 min)

• Stateless functional tests
• Core functionality coverage
• No external dependencies

Generates Report

• Pass/fail counts
• Detailed logs
• Artifacts for debugging