tests: Clear out unused pNext AppInfo field.

Saw segfault with APIDump due to garbage pNext field
diff --git a/tests/test_environment.cpp b/tests/test_environment.cpp
index 7ba7f69..85da914 100644
--- a/tests/test_environment.cpp
+++ b/tests/test_environment.cpp
@@ -15,6 +15,7 @@
     app_.pEngineName = "vk_testing";
     app_.engineVersion = 1;
     app_.apiVersion = VK_API_VERSION;
+    app_.pNext = NULL;
 }
 
 bool Environment::parse_args(int argc, char **argv)