test: improve test-child-process-fork-and-spawn

* use const instead of var for required modules
* use assert.strictEqual instead of assert.equal
* remove unnecessary process.nextTick

PR-URL: nodejs#10273
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

test/parallel/test-child-process-fork-and-spawn.js

@@ -1,8 +1,8 @@
 'use strict';
 const common = require('../common');
-var assert = require('assert');
-var spawn = require('child_process').spawn;
-var fork = require('child_process').fork;
+const assert = require('assert');
+const spawn = require('child_process').spawn;
+const fork = require('child_process').fork;
 
 // Fork, then spawn. The spawned process should not hang.
 switch (process.argv[2] || '') {
@@ -16,10 +16,9 @@ switch (process.argv[2] || '') {
   case 'spawn':
     break;
   default:
-    assert(0);
+    common.fail();
 }
 
 function checkExit(statusCode) {
-  assert.equal(statusCode, 0);
-  process.nextTick(process.exit);
+  assert.strictEqual(statusCode, 0);
 }