While #45610 was positively reviewed by Evan, Ray suggested (via Jabber)
another improvement for lib/dns/tests/rdata_test.c which I think is
sound, but will require a bit more work in #45610 (and thus some more
reviewing).  As preparing DOA RR unit tests has higher priority, I
decided to push them to the rt45612 branch so that they can be reviewed,
resorting to copy-and-paste (which is basically how tests have been
added to lib/dns/tests/rdata_test.c for a while now).  When it is
decided that DOA RR support is good to be merged into master, I will
rebase it against refactored lib/dns/tests/rdata_test.c, which should
already be merged by then.