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.

contentbank/tests/contenttype_test.php

@@ -657,7 +657,7 @@ class contenttype_test extends \advanced_testcase {
     /**
      * Tests pluginfile result.
      *
-     * @covers ::pluginfile
+     * @covers ::__construct
      */
     public function test_pluginfile(): void {
         $this->resetAfterTest();