autotest: add wifi tests for roaming on resume

These test various flavors of roaming on resume.  Check both WPA and RSN
because the WPA case seems to be failing randomly with MIC errors during
the GTK handshake.  Explicitly check roaming within the same band and
between bands.

BUG=chromium-os:10733
TEST=manual:run tests and cross-check results against debug logs

Review URL: http://codereview.chromium.org/6508009

Change-Id: Ifa093ebf08c50b9afc18dffaaae7a82515f55f71
4 files changed