on a closer look, it just doesn't seem to be necessary if we are okay
with just "hiding" records under a delegation NS for normal (ie. non
GLUE_OK) lookups, instead of throwing an exception.

Essentially only one existing test case (which expects an exception
in that setup) should be updated (and the "brokenns2" case just seems
to be redundant and so was removed), and the disabled tests
in zone_finder_context_unittest now pass.