Test infrastructure for the qp-trie

Test infrastructure for the qp-trie

This change adds a number of support routines for the unit tests, and
for benchmarks and fuzz tests to be added later. It isn't necessary to
include the support routines in libdns, since they are not needed by
BIND's installed programs. So `libtest` seems like the best place for
them.

The tests themselves verify that dns_qpkey_fromname() behaves as
expected.