pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a1bc4d3590b1f620485c3ec5290dc628e62476f8

Modified Files
--------------
contrib/pg_freespacemap/.gitignore                 |  4 +
contrib/pg_freespacemap/Makefile                   |  2 +
.../pg_freespacemap/expected/pg_freespacemap.out   | 85 ++++++++++++++++++++++
contrib/pg_freespacemap/sql/pg_freespacemap.sql    | 32 ++++++++
4 files changed, 123 insertions(+)