test-memory: destroy response to free socket

Starting from Node 12, the test started to fail because of
intermittent socket errors, such as ECONNRESET and "socket hang up".

Destroying the response before triggering a new request resolves it.

test/test-memory.js

@@ -83,7 +83,8 @@ describe('memory usage', function() {
         cors_anywhere_child.send(null);
         return;
       }
-      http.request(request, function() {
+      http.request(request, function(res) {
+        res.destroy();
         requestAgain();
       }).on('error', function(error) {
         done(error);