selftests: net: fix tcp listener handling in pmtu.sh

The pmtu.sh test uses a few TCP listener in a problematic way:
It hard-codes a constant timeout to wait for the listener starting-up
in background. That introduces unneeded latency and on very slow and
busy host it can fail.

Additionally the test starts again the same listener in the same
namespace on the same port, just after the previous connection
completed. Fast host can attempt starting the new server before the
old one really closed the socket.

Address the issues using the wait_local_port_listen helper and
explicitly waiting for the background listener process exit.

Fixes: 136a1b43

 ("selftests: net: test vxlan pmtu exceptions with tcp")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/f8e8f6d44427d8c45e9f6a71ee1a321047452087.1706812005.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>