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
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e25e5f7fc6b74c9d4ce82627e9145ef5537412e2

Modified Files
--------------
src/test/perl/PostgreSQL/Test/Cluster.pm    | 15 ++++++++++++++
src/test/recovery/t/019_replslot_limit.pl   | 31 ++++++-----------------------
src/test/recovery/t/033_replay_tsp_drops.pl | 14 ++-----------
3 files changed, 23 insertions(+), 37 deletions(-)