pgsql: Add test module injection_points

Add test module injection_points

This provides basic coverage for injection points within a single
process, while providing some callbacks that can be used for other
tests.  There are plans to extend this module later with more
advanced capabilities for tests.

Author: Michael Paquier, with comment fixes from Ashutosh Bapat.
Reviewed-by: Ashutosh Bapat, Nathan Bossart, Álvaro Herrera, Dilip
Kumar, Amul Sul, Nazir Bilal Yavuz
Discussion: https://postgr.es/m/ZTiV8tn_MIb_H2rE@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/49cd2b93d7dbceefdf9a71cc301d284a2dd234c3

Modified Files
--------------
src/test/modules/Makefile                          |   7 ++
src/test/modules/injection_points/.gitignore       |   4 +
src/test/modules/injection_points/Makefile         |  20 ++++
.../injection_points/expected/injection_points.out | 118 +++++++++++++++++++++
.../injection_points/injection_points--1.0.sql     |  35 ++++++
.../modules/injection_points/injection_points.c    |  95 +++++++++++++++++
.../injection_points/injection_points.control      |   4 +
src/test/modules/injection_points/meson.build      |  37 +++++++
.../injection_points/sql/injection_points.sql      |  34 ++++++
src/test/modules/meson.build                       |   1 +
10 files changed, 355 insertions(+)