test: cleanup test-stdout-close-catch.js

Added common.mustCall in child process on 'close' callback
Changed several 'var' statements to 'const' or 'let' where appropriate
Also linting

PR-URL: nodejs#10006
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

test/parallel/test-stdout-close-catch.js

@@ -1,27 +1,30 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var path = require('path');
-var child_process = require('child_process');
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const child_process = require('child_process');
 
-var testScript = path.join(common.fixturesDir, 'catch-stdout-error.js');
+const testScript = path.join(common.fixturesDir, 'catch-stdout-error.js');
 
-var cmd = JSON.stringify(process.execPath) + ' ' +
-          JSON.stringify(testScript) + ' | ' +
-          JSON.stringify(process.execPath) + ' ' +
-          '-pe "process.stdin.on(\'data\' , () => process.exit(1))"';
+const cmd = JSON.stringify(process.execPath) + ' ' +
+            JSON.stringify(testScript) + ' | ' +
+            JSON.stringify(process.execPath) + ' ' +
+            '-pe "process.stdin.on(\'data\' , () => process.exit(1))"';
 
-var child = child_process.exec(cmd);
-var output = '';
-var outputExpect = { 'code': 'EPIPE',
-                     'errno': 'EPIPE',
-                     'syscall': 'write' };
+const child = child_process.exec(cmd);
+let output = '';
+const outputExpect = {
+  code: 'EPIPE',
+  errno: 'EPIPE',
+  syscall: 'write'
+};
 
 child.stderr.on('data', function(c) {
   output += c;
 });
 
-child.on('close', function(code) {
+
+child.on('close', common.mustCall(function(code) {
   try {
     output = JSON.parse(output);
   } catch (er) {
@@ -31,4 +34,4 @@ child.on('close', function(code) {
 
   assert.deepStrictEqual(output, outputExpect);
   console.log('ok');
-});
+}));