autotest: Start avahi on routers for privetd_PrivetSetupFlow

This test browses mDNS records over the WiFi network from the router,
which requires avahi running on the router.  However, avahi is not
started by default.

BUG=None
TEST=Reboot a router (or stop avahi), run this test, observe that it
passes, unlike before.

Change-Id: I28ccb367b2f6958202d8a0ebd66db168408a9766
Reviewed-on: https://chromium-review.googlesource.com/260832
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Zeping Qiu <zqiu@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
2 files changed