Add integration testing with shadow

Closes #40288 (closed)

This adds integration tests to the Gitlab CI for Snowflake using Shadow. At the moment, it pulls the test configuration from an external repository https://github.com/cohosh/shadow-snowflake-minimal

We could add these files to the Snowflake repository in a tests/ directory, just to keep everything in one place. We also currently require a patch for the Snowflake server to ignore any errors returned from attempting to set the IP_BIND_ADDRESS_NO_PORT socket option. This may be ignored in future versions of Shadow, but the Shadow team will first decide on a policy for stub implementations of unsupported features.