test: http2 emitGoAway post shutdown pre destroy

This commit tests use case when emitGoAway is called when client is
shutting down but is not destroyed.

Refs: nodejs/node#14985
PR-URL: nodejs/node#16215
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

test/parallel/test-http2-goaway-opaquedata.js

@@ -29,6 +29,8 @@ server.listen(0, () => {
     assert.deepStrictEqual(code, 1);
     assert.deepStrictEqual(lastStreamID, 0);
     assert.deepStrictEqual(data, buf);
+    // Call shutdown() here so that emitGoaway calls destroy()
+    client.shutdown();
     server.close();
   }));
   const req = client.request({ ':path': '/' });