A test button for each function in the COM Library with a corresponding display. Could also be one test button that calls each function in the COM library and displays the results based on the current value in the edit box. I used the one button - one display approach but you can decide for yourself as long as all functions in the COM interface are tested and the results displayed. I would suggest a separate edit box and button for the LocateFile function. Do not disable your test buttons just because the edit box is empty.
Empty input is a valid test case.
No duplicate code within the same class. Optional additions Add a second interface to the COM Server and the necessary tests in the test driver. There is a wide range of expertise in the class so here is your chance to learn from each other. Be considerate but do not be afraid to ask or offer an answer. I expect each of you to participate but this not a test but rather a classroom discussion.