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/xapi/tests/handler_test.php

@@ -47,7 +47,7 @@ class handler_test extends advanced_testcase {
     /**
      * Test handler creation.
      */
-    public function test_handler_create() {
+    public function test_handler_create(): void {
         // Get an existent handler.
         $handler = handler::create('fake_component');
         $this->assertEquals(get_class($handler), 'fake_component\\xapi\\handler');
@@ -60,7 +60,7 @@ class handler_test extends advanced_testcase {
     /**
      * Test xAPI support.
      */
-    public function test_supports_xapi() {
+    public function test_supports_xapi(): void {
         // Get an existent handler.
         $result = handler::supports_xapi('fake_component');
         $this->assertTrue($result);
@@ -73,7 +73,7 @@ class handler_test extends advanced_testcase {
     /**
      * Test support group.
      */
-    public function test_support_group_actor() {
+    public function test_support_group_actor(): void {
         global $CFG;
         // Get an existent handler.
         $this->resetAfterTest();
@@ -86,7 +86,7 @@ class handler_test extends advanced_testcase {
     /**
      * Test for process_statements method.
      */
-    public function test_process_statements() {
+    public function test_process_statements(): void {
 
         $this->resetAfterTest();
         $this->preventResetByRollback(); // Logging waits till the transaction gets committed.