tests: Test Non-API headers

Ensures the non-API headers compile correctly

tests/vk_layer.c

@@ -0,0 +1,7 @@
+#include "vulkan/vk_layer.h"
+
+// Use helper macro intended for vulkan layers to export functions
+VK_LAYER_EXPORT int foobar()
+{
+    return 0;
+}