test(agc): skip agc_driver_prepared_shaders_deferred_undecodable without a Vulkan device

The --deferred-undecodable mode registers shaders through a Vulkan
device but, unlike --deferred, did not check for one first. On the
Windows CI runner, which has no Vulkan driver, vkCreateInstance returns
VK_ERROR_INCOMPATIBLE_DRIVER and the test fails instead of skipping, so
the windows job on main has failed since e5b32bb4. It now returns
VulkanTestSkipped like the other modes.

core/libs/prx/libSceAgcDriver/tests/execution/PreparedShaders.cpp

@@ -484,6 +484,7 @@ int main(int argc, char** argv) {
             return 0;
         }
         if (argc == 2 && std::string_view(argv[1]) == "--deferred-undecodable") {
+            if (!OpenVulkanTestDevice()) return VulkanTestSkipped;
             DeferredUndecodableRegistration();
             std::cout << "deferred undecodable shader tests passed\n";
             return 0;