Fix test case instability introduced in 085b6b66.

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.

src/test/regress/expected/misc_functions.out

@@ -167,7 +167,8 @@ select count(*) > 0 as ok from (select * from pg_ls_waldir() limit 1) ss;
  t
 (1 row)
 
-select (pg_ls_waldir()).size = :segsize as ok limit 1;
+select (w).size = :segsize as ok
+from (select pg_ls_waldir() w) ss where length((w).name) = 24 limit 1;
  ok 
 ----
  t

src/test/regress/sql/misc_functions.sql

@@ -46,7 +46,8 @@ 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 = :segsize as ok limit 1;
+select (w).size = :segsize as ok
+from (select pg_ls_waldir() w) ss where length((w).name) = 24 limit 1;
 
 select count(*) >= 0 as ok from pg_ls_archive_statusdir();