tests: disable pg capture before enabling it

tests: disable pg capture before enabling it

In a lot of places within the unit tests pg_start() starts
the capture with an already existing capture running
for the same test.

If the pcap file already exists, then it is renamed and there
is no problem.

However, there is a potential for race if the previous
pg_start() has enabled the capture, but the check for
renaming it happened just slightly earlier than the first
packet has arrived.

Then a second call to pg_start() will hit a check that
a file exists, and will cause an error. This is especially
visible when running the tests in parallel due to increased
load.

Solution: disable the capture before enabling it.
This will flush the aready running capture and eliminate the race.

The additional delay that flushing of the pcap creates has exposed
several other race conditions:

NAT tests: Some of the NAT reassembly tests
verify that the entries were added to the reassembly data structures,
but do so by comparing the quantities of entries. With the default
timeout being 2s, some of the entries might timeout,
resulting in a bogus test failure.

Solution: Bump the timeout to 20s for the affected tests.

Punt tests: nr_packets == 3 makes test intermittently fail,
nr_packets > 3 make it reliably fail, and nr_packets = 2 works

Solution: set nr_packets == 2 for the time being

IGMP tests: the leave-group calls get a spurious packet
from the time the new groups were configured

Solution: add 1 second delay before starting to delete the groups

Type: test

Change-Id: I931182a7b2860cf670e030ee7da8038f6e87356d
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>