Fix ODR violations in tests

CMake builds all tests into a single executable. So, naming these two
classes the same is an ODR violation, which also causes segmentation
fault on my machine when I try to run tests.

With this patch, test executable do not crash anymore. However, there
are still 13 failing tests, which require further investigation.

tests/TensorProductPatchTest.cpp

@@ -265,7 +265,7 @@ TEST_F(TensorProductPatchTest, bbox) {
 }
 
 
-class Callback : public ShadingPatch::Callback {
+class TensorProductPatchTestCallback : public ShadingPatch::Callback {
 	public:
 		void patchSegment (GraphicsPath<double> &path, const Color &color) {
 			ostringstream oss;
@@ -284,7 +284,7 @@ class Callback : public ShadingPatch::Callback {
 
 
 TEST_F(TensorProductPatchTest, approximate) {
-	Callback callback;
+	TensorProductPatchTestCallback callback;
 	vector<Color> colors(4);
 	TensorProductPatch tpp(_points, colors, Color::ColorSpace::RGB, 0, 0);
 	tpp.approximate(2, false, 0.1, callback);

tests/TriangularPatchTest.cpp

@@ -142,7 +142,7 @@ TEST(TriangularPatchTest, boundaryPath) {
 }
 
 
-class Callback : public ShadingPatch::Callback {
+class TriangularPatchTestCallback : public ShadingPatch::Callback {
 	public:
 		void patchSegment (GraphicsPath<double> &path, const Color &color) {
 			ostringstream oss;
@@ -167,7 +167,7 @@ TEST(TriangularPatchTest, approximate) {
 	points[2] = DPair(0, 10);
 	vector<Color> colors(3);
 	TriangularPatch tp(points, colors, Color::ColorSpace::RGB, 0, 0);
-	Callback callback;
+	TriangularPatchTestCallback callback;
 	tp.approximate(2, false, 0.1, callback);
 	EXPECT_EQ(callback.pathstr(), "M0 0H10L0 10Z");
 	EXPECT_EQ(callback.colorstr(), "#000");