amcheck: Add a GIN index to the CREATE INDEX CONCURRENTLY tests

amcheck: Add a GIN index to the CREATE INDEX CONCURRENTLY tests

The existing CREATE INDEX CONCURRENTLY tests checking only B-Tree, but
can be cheaply extended to also check GIN. This helps increasing test
coverage for GIN amcheck, especially related to handling concurrent page
splits and posting list trees.

This already helped to identify several issues during development of the
GIN amcheck support.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-By: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-By: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/BC221A56-977C-418E-A1B8-9EFC881D80C5%40enterprisedb.com