fixing working with test data and a small fix for init code

- set init value for `numsdev` to prevent use of uninitialized value
- stop use of 'workdir' and files from samples
- forcing use of 'opencv_extra' instead
Note: set OPENCV_TEST_DATA_PATH to full path to 'opencv_extra/testdata' (gitolite@code.opencv.org:opencv_extra.git) before running the test!

modules/ocl/test/test_canny.cpp

@@ -48,7 +48,6 @@
 
 ////////////////////////////////////////////////////////
 // Canny
-extern std::string workdir;
 IMPLEMENT_PARAM_CLASS(AppertureSize, int);
 IMPLEMENT_PARAM_CLASS(L2gradient, bool);
 
@@ -67,7 +66,7 @@ PARAM_TEST_CASE(Canny, AppertureSize, L2gradient)
 
 TEST_P(Canny, Accuracy)
 {
-    cv::Mat img = readImage(workdir + "fruits.jpg", cv::IMREAD_GRAYSCALE);
+    cv::Mat img = readImage("cv/shared/fruits.png", cv::IMREAD_GRAYSCALE);
     ASSERT_FALSE(img.empty());
 
     double low_thresh = 50.0;