autotest: Make WiFiClient an instance of LinuxSystem

This lets us consolidate some packet capture code and enables
us to use dual radio stumpies as both packet capture machines
and WiFiClients at the same time.

BUG=chromium:308214
TEST=wifi_matfunc still passes with these changes.
Taking packet captures with this modified WiFiClient still works.

Change-Id: Id143357f63be5ef2c8914cf0ff295408ab9b143b
Reviewed-on: https://chromium-review.googlesource.com/180097
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
6 files changed