test: refactor test-child-process-kill

* var -> const
* assert.equal() -> assert.strictEqual()

PR-URL: https://github.com/nodejs/node/pull/9903
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/parallel/test-child-process-kill.js

@@ -1,8 +1,8 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var spawn = require('child_process').spawn;
-var cat = spawn(common.isWindows ? 'cmd' : 'cat');
+const common = require('../common');
+const assert = require('assert');
+const spawn = require('child_process').spawn;
+const cat = spawn(common.isWindows ? 'cmd' : 'cat');
 
 cat.stdout.on('end', common.mustCall(function() {}));
 cat.stderr.on('data', common.fail);
@@ -13,6 +13,6 @@ cat.on('exit', common.mustCall(function(code, signal) {
   assert.strictEqual(signal, 'SIGTERM');
 }));
 
-assert.equal(cat.killed, false);
+assert.strictEqual(cat.killed, false);
 cat.kill();
-assert.equal(cat.killed, true);
+assert.strictEqual(cat.killed, true);