mysqld runner for tests

Changes for 1.0020 - 2024-04-05T16:35:54Z

• introduce GitHub Actions for CI
• Use UNIX socket authentication with MariaDB 10.4.3
• ignore-db-dir on MySQL 5.7 only
• Don't loop endlessly if mysqld fails to start
• Replace which with command -v for POSIX-compliant
• fix Test 05-copy-data-from.t Fails