test: improve code in test-fs-readfile-error

* use const instead of var
* use common.mustCall to control the functions execution automatically
* use assert.strictEqual instead of assert.equal
* use assert.notStrictEqual instead of assert.notEqual
* use arrow functions

PR-URL: #10367
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-fs-readfile-error.js

@@ -1,8 +1,8 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var exec = require('child_process').exec;
-var path = require('path');
+const common = require('../common');
+const assert = require('assert');
+const exec = require('child_process').exec;
+const path = require('path');
 
 // `fs.readFile('/')` does not fail on FreeBSD, because you can open and read
 // the directory there.
@@ -14,28 +14,28 @@ if (process.platform === 'freebsd') {
 var callbacks = 0;
 
 function test(env, cb) {
-  var filename = path.join(common.fixturesDir, 'test-fs-readfile-error.js');
-  var execPath = '"' + process.execPath + '" "' + filename + '"';
-  var options = { env: Object.assign(process.env, env) };
-  exec(execPath, options, function(err, stdout, stderr) {
+  const filename = path.join(common.fixturesDir, 'test-fs-readfile-error.js');
+  const execPath = '"' + process.execPath + '" "' + filename + '"';
+  const options = { env: Object.assign(process.env, env) };
+  exec(execPath, options, common.mustCall((err, stdout, stderr) => {
     assert(err);
-    assert.equal(stdout, '');
-    assert.notEqual(stderr, '');
+    assert.strictEqual(stdout, '');
+    assert.notStrictEqual(stderr, '');
     cb('' + stderr);
-  });
+  }));
 }
 
-test({ NODE_DEBUG: '' }, function(data) {
+test({ NODE_DEBUG: '' }, common.mustCall((data) => {
   assert(/EISDIR/.test(data));
   assert(!/test-fs-readfile-error/.test(data));
   callbacks++;
-});
+}));
 
-test({ NODE_DEBUG: 'fs' }, function(data) {
+test({ NODE_DEBUG: 'fs' }, common.mustCall((data) => {
   assert(/EISDIR/.test(data));
   assert(/test-fs-readfile-error/.test(data));
   callbacks++;
-});
+}));
 
 process.on('exit', function() {
   assert.equal(callbacks, 2);