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/block_test.php

@@ -32,8 +32,6 @@ class block_test extends advanced_testcase {
 
     /**
      * Test the get_enabled_plugins method.
-     *
-     * @covers ::get_enabled_plugins
      */
     public function test_get_enabled_plugins(): void {
         $this->resetAfterTest();