MDL-75880 phpunit: fix all @coversDefaultClass missing @Covers

Because @coversDefaultClass doesn't indicate any coverage (it's
just an alias to avoid having to write the class name in @Covers),
this commit fixes all the files that were using that annotation
and missing any @Covers.

Basically, replacing one by the other and done.

lib/table/tests/local/filter/filter_test.php

@@ -14,15 +14,6 @@
 // You should have received a copy of the GNU General Public License
 // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
 
-/**
- * Unit tests for core_table\local\filter\filter.
- *
- * @package   core_table
- * @category  test
- * @copyright 2020 Andrew Nicols <andrew@nicols.co.uk>
- * @license   http://www.gnu.org/copyleft/gpl.html GNU Public License
- */
-
 declare(strict_types=1);
 
 namespace core_table\local\filter;
@@ -33,11 +24,11 @@
 /**
  * Unit tests for core_table\local\filter\filter.
  *
- * @coversDefaultClass \core_table\local\filter\filter
  * @package   core_table
  * @category  test
  * @copyright 2020 Andrew Nicols <andrew@nicols.co.uk>
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers    \core_table\local\filter\filter
  */
 class filter_test extends advanced_testcase {
     /**

lib/tests/oauth2_test.php

@@ -30,7 +30,7 @@
  * @package    core
  * @copyright  2017 Damyon Wiese
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later.
- * @coversDefaultClass \core\oauth2\api
+ * @covers     \core\oauth2\api
  */
 class core_oauth2_testcase extends advanced_testcase {