EZP-21434: Fixed file names for fixtures

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`

Cheers
:octocat: Jérôme