selftests: openvswitch: fix tcpdump execution

Fix the way tcpdump is executed by:
- Using the right variable for the namespace. Currently the use of the
  empty "ns" makes the command fail.
- Waiting until it starts to capture to ensure the interesting traffic
  is caught on slow systems.
- Using line-buffered output to ensure logs are available when the test
  is paused with "-p". Otherwise the last chunk of data might only be
  written when tcpdump is killed.

Fixes: 74cc26f4 ("selftests: openvswitch: add interface support")
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Link: https://patch.msgid.link/20241217211652.483016-1-amorenoz@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>