WifiManagerTest#testWifiWatchdog: Disambiguate failures

There is nothing in the WifiWatchdog test that supports the
assumption that the device under test is pre-configured with a
working WiFi network configuration.  This CL makes it possible
from the test stack trace whether the test failed at the
beginning or during the reconnections.

Bug: 27638295
Change-Id: Iff1bd398c74076fb1a969741818562ea582e868e
1 file changed