Fix and activate snowflake tests in CI

[windiana42]

In fact snowflake tests never ran successfully in CI. Even though there is a test group that is called like this, it did not run the tests. I got tests running locally on the pixi migration branch. However, I failed to make it pass in CI. In order to get pixi migration through the door, we decided to deactivate snowflake tests as they were also not active before.

Snowflake tests have additional challenges since the snowflake-sqlalchemy adapter requires pandas<2 and sqlalchemy<2 dependencies. We want to support both, but it is not the main-stream usage of pipedag.