Fix test case instability introduced in 085b6b667.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: a029a0641cce002acbb5e8ac6ec5533e599a9aea
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2020-03-11T22:24:11Z
Releases: 13.0
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.

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