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

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -2742,6 +2742,29 @@ sub lsn
 
 =pod
 
+=item $node->wait_for_event(wait_event_name, backend_type)
+
+Poll pg_stat_activity until backend_type reaches wait_event_name.
+
+=cut
+
+sub wait_for_event
+{
+	my ($self, $backend_type, $wait_event_name) = @_;
+
+	$self->poll_query_until(
+		'postgres', qq[
+		SELECT count(*) > 0 FROM pg_stat_activity
+		WHERE backend_type = '$backend_type' AND wait_event = '$wait_event_name'
+	])
+	  or die
+	  qq(timed out when waiting for $backend_type to reach wait event '$wait_event_name');
+
+	return;
+}
+
+=pod
+
 =item $node->wait_for_catchup(standby_name, mode, target_lsn)
 
 Wait for the replication connection with application_name standby_name until