This test module was not getting invoked, other than at compile time,
limiting its usefulness -- and keeping its coverage at 0%.  Add a
minimal regression test to ensure it runs on make check-world; this
makes it 92% covered (line-wise), which seems sufficient.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20190529193256.GA17603@alvherre.pgsql