selftests: net: cut more slack for gro fwd tests.

The udpgro_fwd.sh self-tests are somewhat unstable. There are
a few timing constraints the we struggle to meet on very slow
environments.

Instead of skipping the whole tests in such envs, increase the
test resilience WRT very slow hosts: increase the inter-packets
timeouts, avoid resetting the counters every second and finally
disable reduce the background traffic noise.

Tested with:

for I in $(seq 1 100); do
	./tools/testing/selftests/kselftest_install/run_kselftest.sh \
		-t net:udpgro_fwd.sh || exit -1
done

in a slow environment.

Fixes: a062260a

 ("selftests: net: add UDP GRO forwarding self-tests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/f4b6b11064a0d39182a9ae6a853abae3e9b4426a.1706812005.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>