test: Test ALPN enforcement in TLS connections

This commit adds tests to verify ALPN (Application-Layer Protocol Negotiation)
enforcement in TLS connections with Gitaly servers. ALPN enforcement ensures
that servers and clients properly negotiate protocol versions, which improves
security by preventing protocol downgrade attacks.

The tests validate both positive and negative scenarios across different
client-server combinations, including:
- Clients with and without ALPN capabilities
- Server implementations (both standard Gitaly and non-ALPN dummy servers)
- Environment-controlled enforcement behaviors

By testing these combinations, we ensure that connections correctly enforce
ALPN requirements when the enforcement flag is enabled while maintaining
backward compatibility where needed.