Hi Matt!
The Form Submission Data report is particularly difficult to filter test data out of since Insight treats the field data you are viewing as metrics and a raw data dump - so the typical qualitative conditions based off field values (First Name does not contain Test) don't quite work the way you'd like or consistently.
The trick is to add in Contact Attributes using an Analyzer license and filter based off those fields - add them to the report (not the grid) so they aren't visible but you can still leverage when adding a condition. Using the email address field works pretty reliably. So if you can have your internal testers use some consistency in the test email provided (contains the word test or your company's domain) adding a condition filter excluding those values should remove them every time.
Hope that helps and if you have any other questions please let me know!