Code to enable TABLE returns testing. Unsure where to put

[wphilips53]

Followed the design pattern of _returns(). Will allow for testing of table returns in conjunction with changes to tap_funky

[theory]

All code to be shipped with pgTAP should go into sql/pgtap.sql.in and in sql/pgtap--1.2.0--1.2.1.sql. If you change any existing functions (as it appears you do in #295), they should also be changed in sql/pgtap.sql.in and in sql/pgtap--1.2.0--1.2.1.sql.

Would you be able to combine this work into a single pull request, and ideally add some test cases?