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.

bt5/erp5_ui_test/PathTemplateItem/portal_tests/relation_field_zuite/testSimpleRelationFieldSearchDialogLongUrl.zpt

@@ -99,6 +99,11 @@
   <td>//input[@type="checkbox"]</td>
   <td></td>
 </tr>
+<tr>
+  <td>storeText</td>
+  <td>//input[@type="checkbox"]/../../td[2]/a</td>
+  <td>first_cell_text</td>
+</tr>
 <tr>
   <td>clickAndWait</td>
   <td>id=dialog_submit_button</td>
@@ -107,7 +112,7 @@
 <tr>
   <td>verifyValue</td>
   <td>field_my_foo_category_title</td>
-  <td>a2</td>
+  <td>${first_cell_text}</td>
 </tr>
 
   </tbody></table>