pgsql: Increase test coverage for worker_spi by %

Increase test coverage for worker_spi by ∞%

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4b3f1dd71b74f82dedbf37c331d0a444368c4009

Modified Files
--------------
src/test/modules/worker_spi/Makefile | 8 ++++
src/test/modules/worker_spi/dynamic.conf | 2 +
.../modules/worker_spi/expected/worker_spi.out | 50 ++++++++++++++++++++++
src/test/modules/worker_spi/sql/worker_spi.sql | 35 +++++++++++++++
src/test/modules/worker_spi/worker_spi.c | 12 +++++-
5 files changed, 106 insertions(+), 1 deletion(-)