This includes more tests dedicated to relation options, bringing the
coverage of this code close to 100%, and the module can be used for
other purposes, like a base template for an index AM implementation.

Author: Nikolay Sharplov, Michael Paquier
Reviewed-by: Álvaro Herrera, Dent John
Discussion: https://postgr.es/m/17071942.m9zZutALE6@x200m