pgsql: Refactor routine to find single log content pattern in TAP tests

Refactor routine to find single log content pattern in TAP tests

The same routine to check if a specific pattern can be found in the
server logs was copied over four different test scripts.  This refactors
the whole to use a single routine located in PostgreSQL::Test::Cluster,
named log_contains, to grab the contents of the server logs and check
for a specific pattern.

On HEAD, the code previously used assumed that slurp_file() could not
handle an undefined offset, setting it to zero, but slurp_file() does
do an extra fseek() before retrieving the log contents only if an offset
is defined.  In two places, the test was retrieving the full log
contents with slurp_file() after calling substr() to apply an offset,
ignoring that slurp_file() would be able to handle that.

Backpatch all the way down to ease the introduction of new tests that
could rely on the new routine.

Author: Vignesh C
Reviewed-by: Andrew Dunstan, Dagfinn Ilmari Mannsåker, Michael Paquier
Discussion: https://postgr.es/m/CALDaNm0YSiLpjCmajwLfidQrFOrLNKPQir7s__PeVvh9U3uoTQ@mail.gmail.com
Backpatch-through: 11

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/392ea0c78fdb6cb92f1af0793f6c2d48526e6fed

Modified Files
--------------
src/test/authentication/t/003_peer.pl              | 17 ++---------
src/test/perl/PostgreSQL/Test/Cluster.pm           | 16 ++++++++++
src/test/recovery/t/019_replslot_limit.pl          | 35 +++++-----------------
src/test/recovery/t/033_replay_tsp_drops.pl        | 13 +-------
.../recovery/t/035_standby_logical_decoding.pl     | 26 +++-------------
5 files changed, 32 insertions(+), 75 deletions(-)