Add PostgreSQL::Test::Cluster::wait_for_event()

Add PostgreSQL::Test::Cluster::wait_for_event()

Per a demand from the author and the reviewer of this commit, this adds
to Cluster.pm a helper routine that can be used to monitor when a
process reaches a wanted wait event.  This can be used in combination
with the module injection_points for the "wait" callback, though it is
not limited to it as this monitors pg_stat_activity for a wait_event and
a backend_type.

Author: Bertrand Drouvot
Reviewed-by: Andrey Borodin
Discussion: https://postgr.es/m/ZeBB4RMPEZ06TcdY@ip-10-97-1-34.eu-west-3.compute.internal