Add test module for SLRUs

Add test module for SLRUs

This commit introduces a basic facility to test SLRUs, in terms of
initialization, page reads, writes, flushes, truncation and deletions,
using SQL wrappers around the APIs of slru.c.  This should be easily
extensible at will, and it can be used as a starting point for someone
willing to implement an external module that makes use of SLRUs (LWLock
tranche registering and SLRU initialization particularly).

As this requires a loaded library, the tests use a custom configuration
file and are disabled under installcheck.

Author: Aleksander Alekseev, Michael Paquier
Reviewed-by: Pavel Borisov, Daniel Gustafsson, Noah Misch, Maxim Orlov
Discussion: https://postgr.es/m/CAJ7c6TOFoWcHOW4BVe3BG_uikCrO9B91ayx9d6rh5JZr_tPESg@mail.gmail.com