Add pg_version() to PostgreSQL::Test::Cluster

_pg_version (version number based on PostgreSQL::Version) is a field
private to Cluster.pm but there was no helper routine to retrieve it
from a Cluster's node.  The same is done for install_path, for example,
and the version object becomes handy when writing tests that need
version-specific handling.

Reviewed-by: Andrew Dunstan, Daniel Gustafsson
Discussion: https://postgr.es/m/YoWfoJTc987tsxpV@paquier.xyz

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

@@ -331,6 +331,20 @@ sub install_path
 
 =pod
 
+=item $node->pg_version()
+
+The version number for the node, from PostgreSQL::Version.
+
+=cut
+
+sub pg_version
+{
+	my ($self) = @_;
+	return $self->{_pg_version};
+}
+
+=pod
+
 =item $node->config_data($option)
 
 Return a string holding configuration data from pg_config, with $option