New method 'assert_cte_equal' to check the output of CTEs

[Somtom]

Problem

Relates to #4

What changed

• New method 'assert_cte_equal' that allows to check the output of a specific CTE in the query you want to test.
• Now you can also pass a 'query' to the 'table_meta'. The 'query_path' will overwrite a 'query' in case both are provided
• The '_get_results' method now accepts a 'query' instead of needing to load the query within the method. In case you created custom Mock Table classes, you need to take this into account.