Ensure correct use of lib/ns/tests/wrap.c

The following discussion from !985 (merged) should be addressed:

• @michal started a discussion: (+1 comment)

  I am confused here, but maybe I just do not understand what the intent is. A comment in lib/ns/tests/wrap.c says that wrap.c is supposed to be used when LD_WRAP is not available. Here, it will be used when LD_WRAP is available. 🤯

  Current master also wraps isc_nmhandle_unref() for other libns unit tests, but I am not sure if that is intentional or a copy-paste issue.

  There is also no else !HAVE_LD_WRAP branch here - not sure if that is an intentional omission.