Add checkTruncatedBeacon WiFi test

This tests that a client can connect to an AP even if the AP is
outputting a malformed, truncated beacon.

CQ-DEPEND=CL:232480
BUG=chrome-os-partner:33662
TEST=Run this test, on both ath9k and WP2 based cells

Change-Id: I3cf9f70fcf6445af250ff65d051816c486aee3f0
Reviewed-on: https://chromium-review.googlesource.com/232610
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
3 files changed