test: camera: Use the CameraManager from the CameraTest base class

Make the camera manager a protected field of the CameraTest class, and
use it instead of CameraManager::instance() in the camera tests. This
prepares for the removal of CameraManager::instance().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

test/camera/buffer_import.cpp

@@ -375,7 +375,7 @@ protected:
 			return TestFail;
 		}
 
-		EventDispatcher *dispatcher = CameraManager::instance()->eventDispatcher();
+		EventDispatcher *dispatcher = cm_->eventDispatcher();
 
 		Timer timer;
 		timer.start(5000);