fix macos test failure by following pattern set in #4496

Author: samayer12

test/fetch/encoding.js

@@ -101,12 +101,14 @@ describe('content-encoding chain limit', () => {
 
   async function setupChainServer (t) {
     const server = createServer({ noDelay: true }, (req, res) => {
+      res.socket.setNoDelay(true)
       const encodingCount = parseInt(req.headers['x-encoding-count'] || '1', 10)
       const encodings = Array(encodingCount).fill('identity').join(', ')
       res.writeHead(200, {
         'Content-Encoding': encodings,
         'Content-Type': 'text/plain'
       })
+      res.flushHeaders()
       res.end('test')
     })
     await once(server.listen(0), 'listening')