ui_test_core: introduce a delay after setFile

We observe in some tests - often with
portal_tests/dms_zuite/testAttachDocument from testFunctionalCore that
sometimes firefox does not submit the form after setFile was used ( at
least, there is no corresponding post request in Z2.log ).

Try to address this by adding a short delay.

bt5/erp5_ui_test_core/SkinTemplateItem/portal_skins/erp5_ui_test_core/user-extensions.js.js

@@ -51,7 +51,10 @@ Selenium.prototype.doSetFile = function(locator, url_and_filename) {
       })
       .then(
         function() {
-          promiseState = 'resolved';
+          // Wait a bit, to address some random failures in tests with firefox.
+          setTimeout(() => {
+            promiseState = 'resolved';
+          }, 1000);
         },
         function(error) {
           console.error(error);