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.
3 jobs for <span class="ref-name">fix/random-setFile-functional-test-failures</span> in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner1

01:02:21

passed ERP5.PerformanceTest-TestRunner1

00:33:19

failed ERP5.UnitTest-TestRunner1

01:59:06