Functions with that kind of naming convention are typically not rules, but are defined in Text File rules. If you search on the string "pega.desktop.showHarness" from Dev studio, select the "All Content" instead of "Name" in the 2nd dropdown from the left in the search results. This will show any rules that contain that string, including the Text File rule it's defined in.
Please note that this type of search (called "Rule Content Search") requires your system's rules to be indexed.
Besides the searching way Kurt mentioned above, you can also try use Fiddler tool when you run the use case. The called java script contains the function will be displayed. You may need to do a browser cache cleanup before to do it.