Fix instability in QNetworkConfigurationManager autotest

Fix instability in QNetworkConfigurationManager autotest

QThread::isFinished() can return false after the finished() signal is
emitted, so test the event loop's timeout() function instead.
Don't compare prescan configurations, as these may be cached by the OS.
It was causing the test to fail on linux if run before any other network
test.

Reviewed-by: mread
(cherry picked from commit e2320ec17446dc6e851fcf4ea2d998177b0d8049)

Change-Id: I35d67294871a35e2e63619f4acb0c3c32caa5eea
Reviewed-on: http://codereview.qt.nokia.com/137
Reviewed-by: Markus Goetz