libpq: Add TAP tests for service files and names

This commit adds a set of regression tests that checks various patterns
with service names and service files, with:
- Service file with no contents, used as default for PGSERVICEFILE to
prevent any lookups at the HOME directory of an environment where the
test is run.
- Service file with valid service name and its section.
- Service file at the root of PGSYSCONFDIR, named pg_service.conf.
- Missing service file.
- Service name defined as a connection parameter or as PGSERVICE.

Note that PGSYSCONFDIR is set to always point at a temporary directory
created by the test, so as we never try to look at SYSCONFDIR.

This set of tests has come up as a useful independent addition while
discussing a patch that adds an equivalent of PGSERVICEFILE as a
connection parameter as there have never been any tests for service
files and service names.  Torsten Foertsch and Ryo Kanbayashi have
provided a basic implementation, that I have expanded to what is
introduced in this commit.

Author: Torsten Foertsch <tfoertsch123@gmail.com>
Author: Ryo Kanbayashi <kanbayashi.dev@gmail.com>
Author: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAKkG4_nCjx3a_F3gyXHSPWxD8Sd8URaM89wey7fG_9g7KBkOCQ@mail.gmail.com

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