test: remove undef NDEBUG from at-exit addons test

The at-exit addons test uses asserts like the other addons tests,
but at-exit is the only one that undefines NDEBUG to make sure
that asserts are enabled. This commit removes the undef for
consistency.

PR-URL: https://github.com/nodejs/node/pull/13998
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/addons/at-exit/binding.cc

@@ -1,4 +1,3 @@
-#undef NDEBUG
 #include <assert.h>
 #include <stdlib.h>
 #include <node.h>