Fix const-correctness issues of InDetTestPixelLayerTool.

Fix const-correctness issues of InDetTestPixelLayerTool.

http://goetz.web.cern.ch/goetz/mutable_2019-07-13.html#InnerDetector_Pixel

nightly/master/2019-07-13T2149:InnerDetector/InDetRecTools/InDetTestPixelLayer/InDetTestPixelLayer/InDetTestPixelLayerTool.h +107

   105	    double m_goodFracCut;
   106	
   107	    mutable double m_cachedFracGood;
   108	
   109	    mutable const Trk::TrackParameters* m_cachedTrkParam;
nightly/master/2019-07-13T2149:InnerDetector/InDetRecTools/InDetTestPixelLayer/InDetTestPixelLayer/InDetTestPixelLayerTool.h +109

   107	    mutable double m_cachedFracGood;
   108	
   109	    mutable const Trk::TrackParameters* m_cachedTrkParam;
   110	
   111	    mutable const Trk::Track* m_cached_track_residual;
nightly/master/2019-07-13T2149:InnerDetector/InDetRecTools/InDetTestPixelLayer/InDetTestPixelLayer/InDetTestPixelLayerTool.h +111

   109	    mutable const Trk::TrackParameters* m_cachedTrkParam;
   110	
   111	    mutable const Trk::Track* m_cached_track_residual;
   112	    mutable PixelLayerResidualPullMap m_residual_map;
   113	  };
nightly/master/2019-07-13T2149:InnerDetector/InDetRecTools/InDetTestPixelLayer/InDetTestPixelLayer/InDetTestPixelLayerTool.h +112

   110	
   111	    mutable const Trk::Track* m_cached_track_residual;
   112	    mutable PixelLayerResidualPullMap m_residual_map;
   113	  };

RunTier0Tests.py test was fine. RunTier0Tests.log