test: Add testColumns
test: Add testColumns

Properties:

 * columnCount returns the number of columns in the result set before ever
   'step'ping, but don't rely on this behavior.

 * When 'step' returns a 'Row', this row and subsequent rows should have the
   same 'columnCount', even if columns are added during the query.

   In other words, the first 'step' locks in the number of columns for the rest
   of this query.

 * For an empty result set, you can use the 'columnCount' after
   step returns Done.

 * Columns may be added while a statement is stepping through the table.

IreneKnapp/direct-sqlite,IreneKnapp/direct-sqlite