autotest: Move packet capture logic to delegate object

Move packet capture logic to a delegate PacketCapture object
so that we can share code between server/router/client packet
captures.

TEST=Ran both:
./run_remote_tests.sh --remote=chromeos1-shelf1-host3 check11b \
                      --args="client_capture=on router_capture=on"

and
./run_remote_tests.sh --remote=chromeos1-shelf1-host3 \
                      network_WiFiMatFunc \
                      --args="test_pat=007*
                      run_options=client_capture_all,router_capture_all"
Noted that reasonable looking packet captures appear.  Tested on both
a stumpy cell with a lumpy DUT and a RSPro cell with a Snow DUT.  Note
that the Snow will fall back to a managed mode capture since it does
not support monitor interfaces.
BUG=chromium:231429

Change-Id: Ib6e21db356f130b7ac5f8b910a5705fa48680a97
Reviewed-on: https://gerrit.chromium.org/gerrit/49385
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
7 files changed