test: use strict mode in global setters test

test-global-setters.js was added in
nodejs#26882 and hasn't been modified
since. It appears that it avoids strict mode so that unscoped
identifiers are treated as globals, but that's true in strict mode too.
(In sloppy mode, an assignment to an undefined identifier will create a
global, but that's not what this test does. In strict mode, those
assignments will throw an error, which would seem to be what we would
want.)

test/parallel/test-global-setters.js

@@ -1,4 +1,4 @@
-/* eslint-disable strict */
+'use strict';
 require('../common');
 const assert = require('assert');
 const _process = require('process');