The PHPUnit tests reference files which don't exist at the requested location. See https://github.com/ezsystems/ezcomments/blob/master/packages/ezcomments_extension/ezextension/ezcomments/tests/ezcomcomment_test.php#L16

The files should be named:

• mysql/schema.sql instead of mysql/mysql.sql
• oracle/schema.sql instead of oracle/oracle.sql
• postgresql/schema.sql instead of postgresql/postgresql.sql