Fix instability with WAL fsync test in stats.sql

Fix instability with WAL fsync test in stats.sql

A backend using wal_sync_method set to "open_sync" or "open_datasync"
would fail the test checking the WAL sync data in pg_stat_io.  These
modes guarantee that a sync is done when WAL is written to disk, and the
data checked by the test is not incremented in this case,
issue_xlog_fsync() doing nothing.

Oversight in commit a051e71e28a1.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0uxwg3xAi4nvdBMJ-zJQEeyg+RotuU+ebM2F6CKmnvaYA@mail.gmail.com