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.)

doc/src/sgml/ref/security_label.sgml

@@ -207,7 +207,7 @@ SECURITY LABEL FOR selinux ON TABLE mytable IS 'system_u:object_r:sepgsql_table_
   <title>See Also</title>
   <simplelist type="inline">
    <member><xref linkend="sepgsql"></member>
-   <member><xref linkend="dummy-seclabel"></member>
+   <member><filename>src/test/modules/dummy_seclabel</filename></member>
   </simplelist>
  </refsect1>
 </refentry>