[core] Add unit test for Source::setPrefetchZoomDelta

Test verifies that tile pyramid will request only 4 tiles for
current zoom level, if Source's setPrefetchZoomDelta is 0.

test/map/map.test.cpp

@@ -21,6 +21,7 @@
 #include <mbgl/style/layers/symbol_layer.hpp>
 #include <mbgl/style/sources/geojson_source.hpp>
 #include <mbgl/style/sources/image_source.hpp>
+#include <mbgl/style/sources/vector_source.hpp>
 #include <mbgl/style/style.hpp>
 #include <mbgl/util/async_task.hpp>
 #include <mbgl/util/color.hpp>
@@ -1119,3 +1120,36 @@ TEST(Map, NoHangOnMissingImage) {
     // The test passes if the following call does not hang.
     test.frontend.render(test.map);
 }
+
+TEST(Map, PrefetchDeltaOverride) {
+    MapTest<> test{1, MapMode::Continuous};
+
+    test.map.getStyle().loadJSON(
+        R"STYLE({
+                "layers": [{
+                    "id": "a",
+                    "type": "fill",
+                    "source": "a",
+                    "source-layer": "a",
+                    "minzoom": 0,
+                    "maxzoom": 24
+                }]
+                })STYLE");
+
+    auto source = std::make_unique<VectorSource>("a", Tileset{{"a/{z}/{x}/{y}"}});
+    source->setPrefetchZoomDelta(0);
+    test.map.getStyle().addSource(std::move(source));
+
+    unsigned requestedTiles = 0u;
+    test.fileSource->tileResponse = [&](const Resource&) {
+        ++requestedTiles;
+        Response res;
+        res.noContent = true;
+        return res;
+    };
+
+    test.map.jumpTo(CameraOptions().withZoom(double(16)));
+    test.observer.didFinishLoadingMapCallback = [&] { test.runLoop.stop(); };
+    test.runLoop.run();
+    EXPECT_EQ(4, requestedTiles);
+}