I think that should be jdbc:postgresql://jena_test - note the double slash.
This makes no difference:
#<JAVA-EXCEPTION com.hp.hpl.jena.db.RDFRDBException: Failure to instantiate DB Driver:PostgreSQL java.sql.SQLException: No suitable driver found for jdbc:postgresql://jena_test {27FEFFAC}>.
But everything works (without double slash) when jar is specified via java -cp