Further cleanup for recent JSON-related commits.

Further cleanup for recent JSON-related commits.

The link commands in test_json_parser/Makefile were a long way
shy of a load, as evidenced by buildfarm failures.  Model them
on pgxs.mk's PROGRAM rule.  (Probably we should have put these
two test programs in different subdirectories so we could
actually use the PROGRAM rule.  But I won't question that
decision today.)