test: fix up internet/test-dns after api change

* The test calls an internal API that changed in commit ca9eb71.

* Trying to reverse-lookup a bogus hostname now returns EINVAL rather
  than the (bogus!) status code ENOTIMP.
cares_wrap: don't set oncomplete property from c++

Don't set the oncomplete property in src/cares_wrap.cc, we can do it
just as easily in lib/dns.js.

Switch two closures to the 'function with _this_ object' model. Makes
it impossible for an overzealous closure to capture too much context
and accidentally hold on to too much memory.