test: refactor test-signal-unregister

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

PR-URL: https://github.com/nodejs/node/pull/9920
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-signal-unregister.js

@@ -1,13 +1,13 @@
 'use strict';
-var common = require('../common');
+const common = require('../common');
-var assert = require('assert');
+const assert = require('assert');
-var spawn = require('child_process').spawn;
+const spawn = require('child_process').spawn;
 
-var child = spawn(process.argv[0], [common.fixturesDir + '/should_exit.js']);
+const child = spawn(process.argv[0], [common.fixturesDir + '/should_exit.js']);
 child.stdout.once('data', function() {
   child.kill('SIGINT');
 });
 child.on('exit', common.mustCall(function(exitCode, signalCode) {
-  assert.equal(exitCode, null);
+  assert.strictEqual(exitCode, null);
-  assert.equal(signalCode, 'SIGINT');
+  assert.strictEqual(signalCode, 'SIGINT');
 }));