skip test case nvlist_send_recv__send_many_fds__dgram

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

lib/libnv/tests/nvlist_send_recv_test.c

@@ -483,6 +483,8 @@ ATF_TC_BODY(nvlist_send_recv__send_many_fds__dgram, tc)
 	size_t len;
 	int error;
 
+	atf_tc_skip("https://bugs.freebsd.org/260891");
+
 	/* size of the largest datagram to send */
 	temp_maxdgram = 16772;
 	len = sizeof(maxdgram);