postgres_fdw - make cached connection functions tests meaningful

Hi,

While working on [1], I got to know that there is a new GUC
debug_invalidate_system_caches_always that has been introduced in v14.
It can be used to switch off cache invalidation in
CLOBBER_CACHE_ALWAYS builds which makes cache sensitive tests stable.
Using this GUC, it is quite possible to make cached connection
management function tests more meaningful by returning original
values(true/false, all the output columns) instead of SELECT 1. Note
that the commit f77717b29 stabilized the tests for those functions -
postgres_fdw_disconnect, postgres_fdw_disconnect_all and
postgres_fdw_get_connections by masking actual return value of the
functions.

Attaching a patch to use the new GUC to make the functions return actual output.

Thoughts?

[1] - https://www.postgresql.org/message-id/CALj2ACVGSQsq68y-LmyXKZzbNVgSgsiVKSzsrVXzVgnsZTN26Q%40mail.gmail.com

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com