QUnit: Disable CORS check to avoid error.

In some environments, QUnit fails with this error:

> Access to XMLHttpRequest at 'file:///wp-admin/admin-ajax.php' from origin 'null' has been blocked by CORS policy: Cross origin requests are only supported for protocol schemes: http, data, chrome, https.

Requesting that endpoing via HTTP would require hardcoding a hostname, which would remove the ability of devs to use whatever hostname they want.

We don't need to worry about CORS in this context, so it's safe to disable it.

Fixes #51446
See gruntjs/grunt-contrib-qunit#158
Props garrett-eclipse

Gruntfile.js

@@ -655,6 +655,16 @@ module.exports = function(grunt) {
 			}
 		},
 		qunit: {
+			options: {
+				puppeteer: {
+					ignoreDefaultArgs: true,
+					args: [
+						"--headless",
+						"--disable-web-security",
+						"--allow-file-access-from-files"
+					]
+				},
+			},
 			files: [
 				'tests/qunit/**/*.html',
 				'!tests/qunit/editor/**'