clean up bin/tests, convert unit tests to ATF

This is an old item on the TODO list whose time has come. Let's clean up bin/tests:

• remove bin/tests/master, it's been superfluous for a long time; it was turned into lib/isc/tests/master_test.c, but the original was never removed
• move dnssec-signzone tests into bin/tests/system/dnssec (if they aren't already there)
• rewrite atomic, db, dst, hashes, mem, names, net, rbt, resolver, sockaddr, tasks and timers as ATF tests (and ideally do something about how slow a few of them are)
• review the remaining tests in bin/tests/*_test.c and see if there are others that should be converted
• remove lib/tests and bin/tests/t_api.pl after all tests using them are converted