Further tweaks applied:

  - use macros to simplify test definitions and aid debugging (Ray),
  - updated outdated EDNS Client Subnet "source generic" test,
  - added RFC snippets for reference (Stephen).

I went through bin/tests/rdata_test.c and did not find anything worth
keeping:

    "-s" prints out memory stats; I could not think of a reasonable way
    to weave this into ATF-based tests; each unit test inside
    lib/dns/tests/rdata_test.c checks for unreleased memory, which is
    IMHO the required minimum; suggestions are welcome if anyone thinks
    we should somehow add memory stats to lib/dns/tests/rdata_test.c,

    "-t" passes a truncated buffer to dns_rdata_fromwire(), "-a" passes
    a buffer with trailing zero bytes; as expected results vary by RR
    type, we can just add explicit unit tests handling these cases,

    "-r" prints out RDATA read from stdin in uncompressed wire form;
    again, not sure how this would fit into ATF-based tests, it seems to
    be more of a debugging aid than a check.

Given the above, I removed bin/tests/rdata_test.c in the final commit to
the rt45610 branch.  It is now ready for review (due to the sheer amount
of changes, it probably makes most sense to review commit by commit).