test: refactor parallel/test-tls-delayed-attach

test: refactor parallel/test-tls-delayed-attach

Added description to the test, replace function with arrow function and
implemented common.mustCall

PR-URL: #19421
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-tls-delayed-attach.js

@@ -24,6 +24,12 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+// This test tries to confirm that a TLS Socket will work as expected even if it
+// is created after the original socket has received some data.
+//
+// Ref: https://github.com/nodejs/node-v0.x-archive/issues/6940
+// Ref: https://github.com/nodejs/node-v0.x-archive/pull/6950
+
 const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const tls = require('tls');
@@ -37,30 +43,30 @@ const options = {
   cert: fixtures.readKey('agent1-cert.pem')
 };
 
-const server = net.createServer(function(c) {
+const server = net.createServer(common.mustCall((c) => {
   setTimeout(function() {
     const s = new tls.TLSSocket(c, {
       isServer: true,
       secureContext: tls.createSecureContext(options)
     });
 
-    s.on('data', function(chunk) {
+    s.on('data', (chunk) => {
       received += chunk;
     });
 
-    s.on('end', function() {
+    s.on('end', common.mustCall(() => {
       server.close();
       s.destroy();
-    });
+    }));
   }, 200);
-}).listen(0, function() {
-  const c = tls.connect(this.address().port, {
+})).listen(0, common.mustCall(() => {
+  const c = tls.connect(server.address().port, {
     rejectUnauthorized: false
-  }, function() {
+  }, () => {
     c.end(sent);
   });
-});
+}));
 
-process.on('exit', function() {
+process.on('exit', () => {
   assert.strictEqual(received, sent);
 });