Add some basic regression tests for pg_freespacemap

Add some basic regression tests for pg_freespacemap

The number of relation pages is tricky to get right in a portable way,
particularly across 32b and 64b builds, but checking after the existence
of the FSM and if there is any space available space should be stable
enough with a minimal number of tuples involved.  This commit introduces
checks on a table with some btree, BRIN and hash indexes, as a first
attempt.

Author: Dong Wook Lee, Fabrízio de Royes Mello, Michael Paquier
Discussion: https://postgr.es/m/CAAcByaJ5KW3bd7fJr=jPEyK8M_UzXJFHHBVuOcBe+JHD8txRyQ@mail.gmail.com