Fix regex searching for page verification failures in tests

Fix regex searching for page verification failures in tests

The test for finding page verification failures in the logfiles
were missing the /m modifier to make sure it anchors to every
newline in the search space buffer, and not just the last one.

Spotted while adding a test for the recently reported issue with
excessive WAL for unlogged relations.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDeGrpZbNZdLjd_T4b43xKEEXZN0HGhkFm-1bkBdyzK7AQ@mail.gmail.com