selftests/bpf: More compatible nc options in test_tc_edt

Out of the three nc implementations widely in use, at least two (BSD netcat
and nmap-ncat) do not support -l combined with -s. Modify the nc invocation
to be accepted by all of them.

Fixes: 7df5e3db

 ("selftests: bpf: tc-bpf flow shaping with EDT")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Peter Oskolkov <posk@google.com>
Link: https://lore.kernel.org/bpf/f5bf07dccd8b552a76c84d49e80b86c5aa071122.1571400024.git.jbenc@redhat.com