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.

completion/tests/progress_test.php

@@ -25,6 +25,7 @@ use completion_completion;
  * @category test
  * @copyright 2017 Mark Nelson <[email protected]>
  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers \core_completion\progress
  */
 class progress_test extends \advanced_testcase {
 
@@ -123,8 +124,6 @@ class progress_test extends \advanced_testcase {
 
     /**
      * Tests that the course progress percentage is returned correctly for various grade to pass settings
-     *
-     * @covers \core_completion\progress::get_course_progress_percentage.
      */
     public function test_course_progress_percentage_completion_state(): void {
         global $DB, $CFG;