Fix some failing QAudioInput integration tests on OSX.

This also meant implementing software volume support, and since
this is the last backend to do so, make that interface pure
virtual again.

In some cases the test needed tweaks.

Change-Id: Ie429863f187b43802cdd4f16d841929e0cb0e729
Reviewed-by: Kurt Korbatits  <kurt.korbatits@nokia.com>
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/multimedia/audio/qaudiodevicefactory.cpp

@@ -106,6 +106,8 @@ public:
     QAudio::State state() const { return QAudio::StoppedState; }
     void setFormat(const QAudioFormat&) {}
     QAudioFormat format() const { return QAudioFormat(); }
+    void setVolume(qreal) {}
+    qreal volume() const {return 1.0f;}
 };
 
 class QNullOutputDevice : public QAbstractAudioOutput