Cleanup snowflake tables after test suite

[machow]

Currently, our snowflake tests generate unique tables (so the test suite can be run multiple times concurrently). However, unlike other backends, we don't clean up the tables. See this code:

https://github.com/machow/siuba/blob/main/siuba/tests/helpers.py#L177-L193

The problem is that for other backends like postgres, we simply teardown containers. For bigquery, we make tables that delete after a day. However, I don't think there are similar moves with snowflake.

We should probably just do the following:

• have the backend track tables added
• have its fixture call a teardown_tables() method, to delete them