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.

backup/moodle2/tests/restore_stepslib_date_test.php

@@ -320,7 +320,8 @@ class restore_stepslib_date_test extends \restore_date_testcase {
     /**
      * Checking that the user completion of an activity relating to the view field does not change
      * when doing a course restore.
-     * @covers ::backup_and_restore
+     * @covers \backup_userscompletion_structure_step
+     * @covers \restore_userscompletion_structure_step
      */
     public function test_usercompletion_view_restore(): void {
         global $DB;