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
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/064e029e60a516dea6566652d729419a51294ac2

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