[autotests] Tests using QSignalSpy should call QVERIFY(spy.isValid());

To make debugging test failures easier, each test that uses QSignalSpy should take advantage of the sanity checks performed by the QSignalSpy constructor, e.g.

QSignalSpy spy(object, SIGNAL(someSignal()));
QVERIFY(spy.isValid());

The practice of using QSignalSpy::isValid() should also be documented in the QTestLib Manual.