src/test/perl/PostgreSQL/Test/*.pm not installed

I noticed that the new Perl test modules are not installed, so if you
try to use PostgreSQL/Test/Cluster.pm in an external test from pgxs, it
fails with the modules not being found.

I see no reason for this other than having overseen it in b235d41d9646,
so I propose the attached (for all branches, naturally.)

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"La gente vulgar sólo piensa en pasar el tiempo;
el que tiene talento, en aprovecharlo"