Add test_dsa module.
This covers basic calls within a single backend process, and also
calling dsa_allocate() or dsa_get_address() while in a different
resource owners. The latter case was fixed by the previous commit.
Discussion: https://www.postgresql.org/message-id/
11b70743-c5f3-3910-8e5b-
dd6c115ff829%40gmail.com