By default, test scores are shown on course certificates. This is a global setting that applies to all courses. It's one of the certificate settings.
Step-by-step instructions
Go to Settings.
In the GENERAL section, click Global settings.
In the CERTIFICATES & TESTS section find Test Scores and choose to hide or show:
To show test scores on certificates, select the check box.
To hide test scores on certificates, clear the check box.
Click Save changes.