Fullstack test test_dscp_marking_packets spawns tcpdump
process inside one fake vm's namespace and then tries to
ping this instance from second one.
After that it checks if tcdump captured any packet marked with
specific DSCP mark.
Ping was done usually only once (1 packet) because
it was done by using vm.wait_until_ping() method, so it could
happen sometimes that ping was send before tcpdump actually
started captuting traffic.
In such case test failed because there was no any packet captured.
This patch changes this by:
1. Start tcpdump async process with block=True, so it should be
already really started before test will go to the next steps,
2. Send always 10 packets instead of (usually) only one.
In addition this patch adds logging of captured tcpdump's stdout
and stderr streams. It may help debugging issues with this test in
(cherry picked from commit 8c2a16796b)