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