test: refactor test-child-process-exec-error

* assert.equal() -> assert.strictEqual()
* regex -> .include()
* Change variable representing a function from `fun` to idiomatic `fn`
* var -> const

PR-URL: nodejs#9780
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>

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

@@ -1,12 +1,12 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var child_process = require('child_process');
+const common = require('../common');
+const assert = require('assert');
+const child_process = require('child_process');
 
-function test(fun, code) {
-  fun('does-not-exist', common.mustCall(function(err) {
-    assert.equal(err.code, code);
-    assert(/does\-not\-exist/.test(err.cmd));
+function test(fn, code) {
+  fn('does-not-exist', common.mustCall(function(err) {
+    assert.strictEqual(err.code, code);
+    assert(err.cmd.includes('does-not-exist'));
   }));
 }