Is there anyway that a certificate can be generated when a student has sucessfully completed a test or assignment.

Done this in a module that wasn't to be used on a CMS/LMS.
Make navigation or add a button that links to a last page, this link is dependent upon your module runtime settings for 'success' in the module. Make this page the certificate and add a print me button using an action.
JAVASCRIPT(javasscript='window.print();') to dump to a printer.
If you want it personalised set a text input field for the user to add their name or collect it in a variable earlier in the module and set the text in a field. You can also use this as a safety net by only having the name appear if the module is completed, else it can show something like 'sample only'.
It isn't very elegant but it was quick and easy to do.