Add ptrchain external test file

ash/tests/ptrchain.rs

@@ -0,0 +1,21 @@
+extern crate ash;
+use ash::vk;
+#[test]
+fn test_ptr_chains() {
+    let mut variable_pointers = vk::PhysicalDeviceVariablePointerFeatures::builder();
+    let mut corner = vk::PhysicalDeviceCornerSampledImageFeaturesNV::builder();
+    let chain = vec![
+        &variable_pointers as *const _ as usize,
+        &corner as *const _ as usize,
+    ];
+    let mut device_create_info = vk::DeviceCreateInfo::builder()
+        .push_next(&mut corner)
+        .push_next(&mut variable_pointers);
+    let chain2: Vec<usize> = unsafe {
+        vk::ptr_chain_iter(&mut device_create_info)
+            .skip(1)
+            .map(|ptr| ptr as usize)
+            .collect()
+    };
+    assert_eq!(chain, chain2);
+}