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

src/test/regress/expected/misc_functions.out

@@ -378,6 +378,29 @@ select * from (select pg_ls_dir('.') a) a where a = 'base' limit 1;
  base
 (1 row)
 
+-- Test missing_ok (second argument)
+select pg_ls_dir('does not exist', false, false); -- error
+ERROR:  could not open directory "does not exist": No such file or directory
+select pg_ls_dir('does not exist', true, false); -- ok
+ pg_ls_dir 
+-----------
+(0 rows)
+
+-- Test include_dot_dirs (third argument)
+select count(*) = 1 as dot_found
+  from pg_ls_dir('.', false, true) as ls where ls = '.';
+ dot_found 
+-----------
+ t
+(1 row)
+
+select count(*) = 1 as dot_found
+  from pg_ls_dir('.', false, false) as ls where ls = '.';
+ dot_found 
+-----------
+ f
+(1 row)
+
 select * from (select (pg_timezone_names()).name) ptn where name='UTC' limit 1;
  name 
 ------

src/test/regress/sql/misc_functions.sql

@@ -124,6 +124,14 @@ from (select pg_ls_waldir() w) ss where length((w).name) = 24 limit 1;
 select count(*) >= 0 as ok from pg_ls_archive_statusdir();
 
 select * from (select pg_ls_dir('.') a) a where a = 'base' limit 1;
+-- Test missing_ok (second argument)
+select pg_ls_dir('does not exist', false, false); -- error
+select pg_ls_dir('does not exist', true, false); -- ok
+-- Test include_dot_dirs (third argument)
+select count(*) = 1 as dot_found
+  from pg_ls_dir('.', false, true) as ls where ls = '.';
+select count(*) = 1 as dot_found
+  from pg_ls_dir('.', false, false) as ls where ls = '.';
 
 select * from (select (pg_timezone_names()).name) ptn where name='UTC' limit 1;