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

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