test: cleanup test-child-process-exec-env.js

Replace equal with strictEqual, use const instead of var and
improve test with use of assert.notStrictEqual

PR-URL: #8600
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>

test/parallel/test-child-process-exec-env.js

@@ -1,22 +1,22 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var exec = require('child_process').exec;
-var success_count = 0;
-var error_count = 0;
-var response = '';
-var child;
+const common = require('../common');
+const assert = require('assert');
+const exec = require('child_process').exec;
+let success_count = 0;
+let error_count = 0;
+let response = '';
+let child;
 
 function after(err, stdout, stderr) {
   if (err) {
     error_count++;
     console.log('error!: ' + err.code);
     console.log('stdout: ' + JSON.stringify(stdout));
     console.log('stderr: ' + JSON.stringify(stderr));
-    assert.equal(false, err.killed);
+    assert.strictEqual(false, err.killed);
   } else {
     success_count++;
-    assert.equal(true, stdout != '');
+    assert.notStrictEqual('', stdout);
   }
 }
 
@@ -33,7 +33,7 @@ child.stdout.on('data', function(chunk) {
 
 process.on('exit', function() {
   console.log('response: ', response);
-  assert.equal(1, success_count);
-  assert.equal(0, error_count);
+  assert.strictEqual(1, success_count);
+  assert.strictEqual(0, error_count);
   assert.ok(response.indexOf('HELLO=WORLD') >= 0);
 });