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