spatialGradient: Add basic perf test

modules/imgproc/perf/perf_spatialgradient.cpp

+#include "perf_precomp.hpp"
+
+using namespace std;
+using namespace cv;
+using namespace perf;
+using namespace testing;
+using std::tr1::make_tuple;
+using std::tr1::get;
+
+typedef std::tr1::tuple<Size, int> Size_Ksize_t;
+typedef perf::TestBaseWithParam<Size_Ksize_t> Size_Ksize;
+
+PERF_TEST_P( Size_Ksize, spatialGradient,
+    Combine(
+        SZ_ALL_HD,
+        Values( 3 )
+    )
+)
+{
+    Size size = std::tr1::get<0>(GetParam());
+    int ksize = std::tr1::get<1>(GetParam());
+
+    Mat src(size, CV_8UC1);
+    Mat dx(size, CV_16SC1);
+    Mat dy(size, CV_16SC1);
+
+    declare.in(src, WARMUP_RNG).out(dx, dy);
+
+    TEST_CYCLE() spatialGradient(src, dx, dy, ksize);
+
+    SANITY_CHECK(dx);
+    SANITY_CHECK(dy);
+}
+