ui_test: fix testSimpleRelationFieldSearchDialogLongUrl random failure

This test was clicking on the first item in a non sorted list and then
checking that the text of the selected element was "a2".

Instead, store the text of the first listbox cell in a variable and use
this in check.
11 jobs for fix/ui_test_testSimpleRelationFieldSearchDialogLongUrl in 0 seconds
Status Job ID Name Coverage
  External
running ERP5.CodingStyleTest-Master

35050:05:57

passed ERP5.CodingStyleTest-TestRunner1

00:54:56

running ERP5.PerformanceTest-Master

35050:05:56

passed ERP5.PerformanceTest-TestRunner1

00:23:31

running ERP5.UnitTest-Master

35050:05:56

running ERP5.UnitTest-Master.Medusa

35050:05:43

failed ERP5.UnitTest-TestRunner1

01:23:32

running SlapOS.Eggs.UnitTest-Master.Python3

35050:05:52

canceled ERP5.UnitTest-TestRunner1

01:19:09

failed ERP5.UnitTest-TestRunner1

04:17:01

canceled ERP5.UnitTest-TestRunner1