I always complete a spreadsheet showing asset number / make & model / serial number / test results and notes (which would include comments about non-standard test conditions and advisory items.) It's not a certificate as such but a record of the information gleaned during testing. If applicable, we would also issue a report for management purposes showing the pass / fail status for all items on a site and our observations about maintenance, testing regime and any overall recommendations.
When testing a single item as a one-off, I write the results on the pass label so that in theory the user or next tester can refer to them, as if there is already an asset log with test results it's unlikely to be updated in the meantime.