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.

blocks/html/tests/block_html_test.php

@@ -31,7 +31,7 @@ class block_html_test extends \advanced_testcase {
      * Tests instance files copying.
      * @covers ::instance_copy
      */
-    public function test_instance_copy() {
+    public function test_instance_copy(): void {
         global $USER;
         $this->resetAfterTest();