knowledge_pad_ui_test: use an RSS from erp5.com for the test

Test should not depend on external websites, we already use erp5.com for
some other tests, so let's use it also for RSS.

/reviewed-on !1028
2 jobs for arnau
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

passed ERP5.UnitTest-Master