Add more regression tests for pg_ls_dir()

Add more regression tests for pg_ls_dir()

This system function was being triggered once in the main regression
test suite to check its SRF configuration, and more in other test
modules but nothing checked the behavior of the options missing_ok and
include_dot_dirs.  This commit adds some tests for both options, to
avoid mistakes if this code is manipulated in the future.

Extracted from a larger patch by the same author, with a few tweaks by
me.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20191227170220.GE12890@telsasoft.com