Make test-http-server-keepalive-req-gc robust to GC heuristics (nodejs#186) (nodejs#189)

* Fix test-http-server-keepalive-req-gc

* Format

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Co-authored-by: Etienne Pierre-Doray <etipdoray@gmail.com>
Co-authored-by: Etienne Pierre-doray <etiennep@chromium.org>

Author: 3 people

test/parallel/test-http-server-keepalive-req-gc.js

@@ -14,9 +14,10 @@ const server = createServer(common.mustCall((req, res) => {
   onGC(req, { ongc: common.mustCall(() => { server.close(); }) });
   req.resume();
   req.on('end', common.mustCall(() => {
-    setImmediate(() => {
+    setImmediate(async () => {
       client.end();
-      global.gc();
+      await global.gc({ type: 'major', execution: 'async' });
+      await global.gc({ type: 'major', execution: 'async' });
     });
   }));
   res.end('hello world');