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/update_api_test.php

@@ -39,7 +39,7 @@ class update_api_test extends \advanced_testcase {
     /**
      * Make sure the $CFG->branch is mapped correctly to the format used by the API.
      */
-    public function test_convert_branch_numbering_format() {
+    public function test_convert_branch_numbering_format(): void {
 
         /** @var \core\update\testable_api $client */
         $client = \core\update\testable_api::client();
@@ -55,7 +55,7 @@ class update_api_test extends \advanced_testcase {
     /**
      * Getting info about particular plugin version.
      */
-    public function test_get_plugin_info() {
+    public function test_get_plugin_info(): void {
 
         $client = \core\update\testable_api::client();
 
@@ -80,7 +80,7 @@ class update_api_test extends \advanced_testcase {
     /**
      * Getting info about the most suitable plugin version for us.
      */
-    public function test_find_plugin() {
+    public function test_find_plugin(): void {
 
         $client = \core\update\testable_api::client();
 
@@ -112,7 +112,7 @@ class update_api_test extends \advanced_testcase {
     /**
      * Validating the pluginfo.php response data.
      */
-    public function test_validate_pluginfo_format() {
+    public function test_validate_pluginfo_format(): void {
 
         $client = \core\update\testable_api::client();