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,