This is a common grievance among many users of ALM, for the exact reason you mention. A manager or some one else is the one who is setting up the test sets and adding tests. They though won't be doing the actual run so it would be more ideal then to alter the field then from this default behavior.
Seen a few things different users try. Some hide the TC_TESTER_NAME field completely in the workflow. And use a UDF user list field to allow the selection of their own responsible tester field. Others will use flow to instead preset the responsible tester based on a different fields value.
One example is using the TC_ACTUAL_TESTER field's value to write to TC_TESTER_NAME (responsible tester).
If you wish to try this route, you will need to add code to the below 2 sub routines:
***TestSet_MoveTo (this one will trigger the field to be populated when you switch from one test set to another)***
***TestSetTests_MoveTo (this will trigger the field to be populated when you switch from one test to another test in a test set)***
In each of those routines can place code to hide the responsible tester:
TestSetTest_Fields.field(''TC_TESTER_NAME'').IsVisible = False
Or if you wished to make the responsible tester equal something else, for example:
TestSetTest_Fields.field(''TC_TESTER_NAME'').value = TestSetTest_Fields.field(''TC_ACTUAL_TESTER'').value
If you hid the default field, one could choose to let the value equal a value that uses a custom user list. That way the manager can assign if they know in advance who they want to do the testing runs. Few different approaches could be taken, just need to be sure to make use of effective code in those 2 sub routines.
Hope this helps.