This is advance preparation for introducing even more test modules; the
easy solution is to add them to contrib, but that's bloated enough that
it seems a good time to think of something different.
Moved modules are dummy_seclabel, test_shm_mq, test_parser and
worker_spi.
(test_decoding was also a candidate, but there was too much opposition
to moving that one. We can always reconsider later.)
Alvaro Herrera