MDL-82207 phpunit: fix various @covers annotations (take#1)

This fixes various (not all) wrong @covers annotations that
are reported as warnings by PHPUnit when it's run with
code-coverage enabled.

When possible, the preferred solution is to change to
@covers at class level, that is the recommended way.

If multiple classes are involved, then a mix of @coversDefaultClass
and @covers at method level are used (always trying to use the
minimum needed patch).

This is the first of a series of issues created as sub-tasks
of MDL-82142.

lib/tests/plugininfo/base_test.php

@@ -27,6 +27,7 @@ use testable_plugininfo_base;
  * @package   core
  * @copyright 2019 Andrew Nicols
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers \core\plugininfo\base
  */
 class base_test extends \advanced_testcase {
 
@@ -277,7 +278,6 @@ class base_test extends \advanced_testcase {
 
     /**
      * Ensure that plugintype_supports_ordering() returns true.
-     * @covers ::plugintype_supports_ordering
      */
     public function test_plugintype_supports_ordering(): void {
         $this->assertFalse(base::plugintype_supports_ordering());