dnn: extra testdata files from OPENCV_DNN_TEST_DATA_PATH env

modules/dnn/test/test_main.cpp

 #include "test_precomp.hpp"
 
-CV_TEST_MAIN("")
+static const char* extraTestDataPath =
+#ifdef WINRT
+        NULL;
+#else
+        getenv("OPENCV_DNN_TEST_DATA_PATH");
+#endif
+
+CV_TEST_MAIN("",
+    extraTestDataPath ? (void)cvtest::addDataSearchPath(extraTestDataPath) : (void)0
+)
 
 namespace cvtest
 {