test: refactor test-cluster-disconnect

Replace `process.once('exit', ...)` with `common.mustCall()`.
Remove unneeded variable in loop declaration.

PR-URL: #11981
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-cluster-disconnect.js

@@ -21,7 +21,7 @@ if (cluster.isWorker) {
  const socket = net.connect(port, '127.0.0.1', () => {
  // buffer result
  let result = '';
- socket.on('data', common.mustCall((chunk) => { result += chunk; }));
+ socket.on('data', (chunk) => { result += chunk; });
 
  // check result
  socket.on('end', common.mustCall(() => {
@@ -34,7 +34,7 @@ if (cluster.isWorker) {
  const testCluster = function(cb) {
  let done = 0;
 
- for (let i = 0, l = servers; i < l; i++) {
+ for (let i = 0; i < servers; i++) {
  testConnection(common.PORT + i, (success) => {
  assert.ok(success);
  done += 1;
@@ -60,40 +60,21 @@ if (cluster.isWorker) {
  }
  };
 
-
- const results = {
- start: 0,
- test: 0,
- disconnect: 0
- };
-
  const test = function(again) {
  //1. start cluster
- startCluster(() => {
- results.start += 1;
-
+ startCluster(common.mustCall(() => {
  //2. test cluster
- testCluster(() => {
- results.test += 1;
-
+ testCluster(common.mustCall(() => {
  //3. disconnect cluster
- cluster.disconnect(() => {
- results.disconnect += 1;
-
+ cluster.disconnect(common.mustCall(() => {
  // run test again to confirm cleanup
  if (again) {
  test();
  }
- });
- });
- });
+ }));
+ }));
+ }));
  };
 
  test(true);
-
- process.once('exit', () => {
- assert.strictEqual(results.start, 2);
- assert.strictEqual(results.test, 2);
- assert.strictEqual(results.disconnect, 2);
- });
 }