Fix tst_QEventLoop::processEventsExcludeSocket test

The testcase always returns the expected result, independently of the
QEventLoop::ExcludeSocketNotifiers flag to processEvents.

In Qt4 the same test uses an intermediate QEventLoop and already runs
it before the QEventLoop::ExcludeSocketNotifiers:

  QEventLoop loop;
  // allow the TCP/IP stack time to loopback the data,
  //  so our socket is ready to read
  QTimer::singleShot(200, &loop, SLOT(quit()));
  loop.exec(QEventLoop::ExcludeSocketNotifiers);

This fixes and improves the test by connecting, processing and
checking the bytesWritten signal for the pending connection socket.

Change-Id: I1b1d2b7b83910c87ba3fe48e29ac9fd585ac62ad
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>