ui_test_core: trigger a change event after setFile

This is what browsers do when user selects a file.
4 jobs for for_testrunner_2
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

failed Wendelin.UnitTest-Master