libpq: Add TAP test for nested service file

libpq: Add TAP test for nested service file

This test corresponds to the case of a "service" defined in a service
file, that libpq is not able to support in parseServiceFile().

This has come up during the review of a patch to add more features in
this area, useful on its own.  Piece extracted from a larger patch by
the same author.

Author: Ryo Kanbayashi <kanbayashi.dev@gmail.com>
Discussion: https://postgr.es/m/Zz2AE7NKKLIZTtEh@paquier.xyz