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/sessionlib_test.php

@@ -30,7 +30,7 @@ class sessionlib_test extends \advanced_testcase {
     /**
      * @covers ::cron_setup_user
      */
-    public function test_cron_setup_user() {
+    public function test_cron_setup_user(): void {
         // This function uses the $GLOBALS super global. Disable the VariableNameLowerCase sniff for this function.
         // phpcs:disable moodle.NamingConventions.ValidVariableName.VariableNameLowerCase
 
@@ -245,7 +245,7 @@ class sessionlib_test extends \advanced_testcase {
      * @param array $config Array of key value config settings
      * @param bool $secure Wether cookies should be secure or not
      */
-    public function test_is_moodle_cookie_secure($config, $secure) {
+    public function test_is_moodle_cookie_secure($config, $secure): void {
         global $CFG;
         $this->resetAfterTest();
         foreach ($config as $key => $value) {
@@ -254,7 +254,7 @@ class sessionlib_test extends \advanced_testcase {
         $this->assertEquals($secure, is_moodle_cookie_secure());
     }
 
-    public function test_sesskey() {
+    public function test_sesskey(): void {
         global $USER;
         $this->resetAfterTest();
 
@@ -281,7 +281,7 @@ class sessionlib_test extends \advanced_testcase {
         $this->assertFalse(sesskey());
     }
 
-    public function test_confirm_sesskey() {
+    public function test_confirm_sesskey(): void {
         $this->resetAfterTest();
 
         $sesskey = sesskey();
@@ -303,7 +303,7 @@ class sessionlib_test extends \advanced_testcase {
         $this->assertFalse(confirm_sesskey());
     }
 
-    public function test_require_sesskey() {
+    public function test_require_sesskey(): void {
         $this->resetAfterTest();
 
         $sesskey = sesskey();