Add test module for Table Access Method

Hi all,

During the PGCon Unconference session about Table Access Method one missing
item pointed out is that currently we lack documentation and examples of
TAM.

So in order to improve things a bit in this area I'm proposing to add a
test module for Table Access Method similar what we already have for Index
Access Method.

This code is based on the "blackhole_am" implemented by Michael Paquier:
https://github.com/michaelpq/pg_plugins/tree/main/blackhole_am

Regards,

-- 
Fabrízio de Royes Mello