Refactor log check logic for connect_ok/fails in PostgreSQL::Test::Cluster

 Refactor log check logic for connect_ok/fails in PostgreSQL::Test::Cluster
 
 This commit refactors a bit the code in charge of checking for log
 patterns when connections fail or succeed, by moving the log pattern
 checks into their own routine, for clarity.  This has come up as
 something to improve while discussing the refactoring of find_in_log().
 
 Backpatch down to 14 where these routines are used, to ease the
 introduction of new tests that could rely on them.
 
 Author: Vignesh C, Michael Paquier
 Discussion: https://postgr.es/m/CALDaNm0YSiLpjCmajwLfidQrFOrLNKPQir7s__PeVvh9U3uoTQ@mail.gmail.com
 Backpatch-through: 14