skip test case nvlist_send_recvsend_many_fdsdgram

If I'm not mistaken, the underlying sendmsg() for nvlist_send() is
failing with ENOBUFS. In turn, nvlist_recv() returns NULL because it
didn't receive the expected number of file descriptors.

Adjusting net.local.dgram.recvspace worked on my local machine, but on
CI the test still fails consistently.

PR: 260891