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.
34 jobs for for_testrunner_1 in 0 seconds
Status Job ID Name Coverage
  External
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failed ERP5.UnitTest-Master.Medusa

passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3