test: refactor test-stdin-from-file.js

change var to const/let
wrap common.mustCall on childProcess.exec callback
remove unneeded fs.unlinkSync()
refactor assert.equal to assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/10012
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

test/parallel/test-stdin-from-file.js

@@ -1,17 +1,17 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var join = require('path').join;
-var childProcess = require('child_process');
-var fs = require('fs');
+const common = require('../common');
+const assert = require('assert');
+const join = require('path').join;
+const childProcess = require('child_process');
+const fs = require('fs');
 
-var stdoutScript = join(common.fixturesDir, 'echo-close-check.js');
-var tmpFile = join(common.tmpDir, 'stdin.txt');
+const stdoutScript = join(common.fixturesDir, 'echo-close-check.js');
+const tmpFile = join(common.tmpDir, 'stdin.txt');
 
-var cmd = '"' + process.argv[0] + '" "' + stdoutScript + '" < "' +
+const cmd = '"' + process.argv[0] + '" "' + stdoutScript + '" < "' +
             tmpFile + '"';
 
-var string = 'abc\nümlaut.\nsomething else\n' +
+const string = 'abc\nümlaut.\nsomething else\n' +
                '南越国是前203年至前111年存在于岭南地区的一个国家，国都位于番禺，' +
                '疆域包括今天中国的广东、广西两省区的大部份地区，福建省、湖南、贵州、' +
                '云南的一小部份地区和越南的北部。南越国是秦朝灭亡后，' +
@@ -28,17 +28,13 @@ common.refreshTmpDir();
 
 console.log(cmd + '\n\n');
 
-try {
-  fs.unlinkSync(tmpFile);
-} catch (e) {}
-
 fs.writeFileSync(tmpFile, string);
 
-childProcess.exec(cmd, function(err, stdout, stderr) {
+childProcess.exec(cmd, common.mustCall(function(err, stdout, stderr) {
   fs.unlinkSync(tmpFile);
 
   if (err) throw err;
   console.log(stdout);
-  assert.equal(stdout, 'hello world\r\n' + string);
-  assert.equal('', stderr);
-});
+  assert.strictEqual(stdout, 'hello world\r\n' + string);
+  assert.strictEqual('', stderr);
+}));