MDL-66979 behat: Work around weird gradebook focus behaviour

I can't expand this one in a sane fashion but the older JS is doing some
weird and wonderful things with focus.

The only way I could get this to behave correctly was to move away from
the field, move back to it, and then move away again. Other combinations
failed in different browsers for different reasons.

grade/report/grader/tests/behat/ajax_grader.feature

@@ -65,6 +65,8 @@ Feature: Using the AJAX grading feature of Grader report to update grades and fe
     And I click on student "Student 3" for grade item "Item SU"
     And I set the field "ajaxgrade" to "Very good"
     And I press the shift tab key
+    And I press the tab key
+    And I press the shift tab key
     And I click on student "Student 3" for grade item "Item VU"
     And I set the field "ajaxgrade" to "50"
     And I press the enter key