MDL-81522 phpunit: Add missing void return type to all tests

While this change is not 100% required now, it's good habit
and we are checking for it since Moodle 4.4.

All the changes in this commit have been applied automatically
using the moodle.PHPUnit.TestReturnType sniff and are, exclusively
adding the ": void" return types when missing.

lib/tests/context_test.php

@@ -31,7 +31,7 @@ class context_test extends \advanced_testcase {
      * Tests legacy class name.
      * @coversNothing
      */
-    public function test_legacy_classname() {
+    public function test_legacy_classname(): void {
         $this->assertSame('core\context', context::class);
 
         $context = \context_system::instance();
@@ -43,7 +43,7 @@ class context_test extends \advanced_testcase {
      * Tests covered method.
      * @covers ::instance_by_id
      */
-    public function test_factory_methods() {
+    public function test_factory_methods(): void {
         $context = context::instance_by_id(SYSCONTEXTID);
         $this->assertSame('core\\context\\system', get_class($context));
     }
@@ -53,7 +53,7 @@ class context_test extends \advanced_testcase {
      * @covers ::__set
      * @covers ::__unset
      */
-    public function test_propery_change_protection() {
+    public function test_propery_change_protection(): void {
         $context = context\system::instance();
 
         $context->contextlevel = -10;
@@ -79,7 +79,7 @@ class context_test extends \advanced_testcase {
      * Tests covered method.
      * @covers ::__get
      */
-    public function test_incorrect_property() {
+    public function test_incorrect_property(): void {
         $context = context\system::instance();
 
         $a = $context->whatever;
@@ -90,7 +90,7 @@ class context_test extends \advanced_testcase {
      * Tests covered method.
      * @covers ::getIterator
      */
-    public function test_iterator() {
+    public function test_iterator(): void {
         $context = context\system::instance();
         $array = iterator_to_array($context->getIterator());
         $expected = [