Move test modules from contrib to src/test/modules

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.)

src/test/modules/test_parser/test_parser.control

@@ -0,0 +1,5 @@
+# test_parser extension
+comment = 'example of a custom parser for full-text search'
+default_version = '1.0'
+module_pathname = '$libdir/test_parser'
+relocatable = true