Complete coverage of DDL/DML tests

Review current DDL/DML tests, filling missing ones and completing them to cover all the stuff and cases.

Some things to review:

testing insert/update/set_field nulls, insert/update/set_field string data in numeric fields, insert/update/set_field boolean data into numeric/varchar/text fields, select/insert/update/set_fieldempty varchar and text fields, lowercase fieldnames, defaults... functions without params... sql_helper() functions..., limits when fetching records, all the get_columns() information, returned id on insert, get_indexes (with some column named, "time" because of PG internals).