Cleanup some code related to pgbench log checks in TAP tests

Cleanup some code related to pgbench log checks in TAP tests

This fixes a couple of problems within the so-said code of this commit
subject:
- Replace the use of open() with slurp_file(), fixing an issue reported
by buildfarm member fairywren whose perl installation keep around CRLF
characters, causing the matching patterns for the logs to fail.
- Remove the eval block, which is not really necessary.

This set of issues has come into light after fixing a different issue
with c13585fe, and this is wrong since this code has been introduced.

Reported-by: Andrew Dunstan, and buildfarm member fairywren
Author: Michael Paquier
Reviewed-by: Andrew Dunstan
Discussion: https://postgr.es/m/0f49303e-7784-b3ee-200b-cbf67be2eb9e@dunslane.net
Backpatch-through: 11