Fix test case instability introduced in 085b6b667.

I forgot that the WAL directory might hold other files besides WAL
segments, notably including new segments still being filled.
That means a blind test for the first file's size being 16MB can
fail.  Restrict based on file name length to make it more robust.

Per buildfarm.

2 files changed, 4 insertions, 2 deletions

diff --git a/src/test/regress/expected/misc_functions.out b/src/test/regress/expected/misc_functions.out
index 8a23c4cc396..76b23049ea4 100644
--- a/src/test/regress/expected/misc_functions.out
+++ b/src/test/regress/expected/misc_functions.out
@@ -170,7 +170,8 @@ select count(*) > 0 as ok from (select * from pg_ls_waldir() limit 1) ss;
  t
 (1 row)
 
-select (pg_ls_waldir()).size = :segbsize * :bsize as ok limit 1;
+select (w).size = :segbsize * :bsize as ok
+from (select pg_ls_waldir() w) ss where length((w).name) = 24 limit 1;
  ok 
 ----
  t
diff --git a/src/test/regress/sql/misc_functions.sql b/src/test/regress/sql/misc_functions.sql
index af834645c0c..cba43c553b9 100644
--- a/src/test/regress/sql/misc_functions.sql
+++ b/src/test/regress/sql/misc_functions.sql
@@ -49,4 +49,5 @@ select count(*) > 0 as ok from (select pg_ls_waldir()) ss;
 -- Test not-run-to-completion cases.
 select * from pg_ls_waldir() limit 0;
 select count(*) > 0 as ok from (select * from pg_ls_waldir() limit 1) ss;
-select (pg_ls_waldir()).size = :segbsize * :bsize as ok limit 1;
+select (w).size = :segbsize * :bsize as ok
+from (select pg_ls_waldir() w) ss where length((w).name) = 24 limit 1;