WopiTestFrame does not work - WOPI host implementation
Solution 1:
The WopiTestFrame.aspx is only present in Office Online - the on-demand version of Office with which you can integrate if you are a member of the Office 365 - Cloud Storage Partner Program.
Though most of the documentation at https://docs.microsoft.com/en-us/microsoft-365/cloud-storage-partner-program/ is common with Office Online Server 2016, it was mainly written for the purposes of the aforementioned integration.