test: remove disabled test-dgram-send-error

This test was disabled in 2013 because it spams random IPs with UDP
messages. We've been doing fine for four years without so let's delete
it.

PR-URL: #12330
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Santiago Gimeno <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>