autotest: Put flaky WiFi tests in a flaky test suite

While we're here, correct the test suite to only run on the most recent
branch.

TEST=None
BUG=None

Change-Id: Ifdac3483f7fa5762fe16426c559c565b3192bd8e
Reviewed-on: https://gerrit.chromium.org/gerrit/61198
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
3 files changed