Thread: pgsql: Fix test case instability introduced in 085b6b667.

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.

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/630590d6ff632b95418eff9e98482d9a93b60ee3

Modified Files
--------------
src/test/regress/expected/misc_functions.out | 3 ++-
src/test/regress/sql/misc_functions.sql      | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)