[FIX] web_editor: adapt test to new transparent.png location

A mockRPC was catching the route to transparent.png file which has been
relocated with [1]. This commit solves that unadapted mockRPC and makes
it easier to grep.

[1]: https://github.com/odoo/odoo/commit/94b59b1972e1a18e18dfcd6c2ded495391380b9a

Part of https://github.com/odoo/odoo/pull/54274

addons/web_editor/static/tests/field_html_tests.js

@@ -253,7 +253,7 @@ QUnit.module('web_editor', {}, function () {
                             return Promise.resolve();
                         }
                     }
-                    if (route.indexOf('/web_editor/static/src/img/') === 0) {
+                    if (route.indexOf('/web/static/src/img/transparent.png') === 0) {
                         return Promise.resolve();
                     }
                     if (route.indexOf('/web_unsplash/fetch_images') === 0) {