cmake: fix build of dnn tests with shared common code

- don't share .cpp files (PCH support is broken)

modules/dnn/perf/perf_common.cpp

@@ -0,0 +1,6 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+
+#include "perf_precomp.hpp"
+#include "../test/test_common.impl.hpp"  // shared with accuracy tests