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/external/tests/external_api_test.php

@@ -332,7 +332,7 @@ class external_api_test extends \advanced_testcase {
     /**
      * Test \core_external\external_api::get_context()_from_params parameter validation.
      *
-     * @covers \core_external\external_api::get_context
+     * @covers \core_external\external_api::get_context_from_params
      */
     public function test_get_context_params(): void {
         global $USER;
@@ -345,7 +345,7 @@ class external_api_test extends \advanced_testcase {
     /**
      * Test \core_external\external_api::get_context()_from_params parameter validation.
      *
-     * @covers \core_external\external_api::get_context
+     * @covers \core_external\external_api::get_context_from_params
      */
     public function test_get_context_params2(): void {
         global $USER;
@@ -357,7 +357,7 @@ class external_api_test extends \advanced_testcase {
 
     /**
      * Test \core_external\external_api::get_context()_from_params parameter validation.
-     * @covers \core_external\external_api::get_context
+     * @covers \core_external\external_api::get_context_from_params
      */
     public function test_get_context_params3(): void {
         global $USER;