tests: speed snowflake tests by removing reflection

[machow]

• removes the use of reflection when loading data for snowflake tests

The problem what that snowflake sqlalchemy getting the columns of a table was taking ~10s. Below is an example query it would generate:

SELECT /* sqlalchemy:_get_schema_columns */
                   ic.table_name,
                   ic.column_name,
                   ic.data_type,
                   ic.character_maximum_length,
                   ic.numeric_precision,
                   ic.numeric_scale,
                   ic.is_nullable,
                   ic.column_default,
                   ic.is_identity,
                   ic.comment
              FROM information_schema.columns ic
             WHERE ic.table_schema='TESTS'
             ORDER BY ic.ordinal_position

(note that this may be related to the number of tables in our database? or could be improved somehow? idk)