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.

mod/data/tests/dates_test.php

@@ -93,7 +93,7 @@ class dates_test extends advanced_testcase {
      * @param int|null $availableto The "available to" time in the database activity.
      * @param array $expected The expected value of calling get_dates_for_module()
      */
-    public function test_get_dates_for_module(?int $availablefrom, ?int $availableto, array $expected) {
+    public function test_get_dates_for_module(?int $availablefrom, ?int $availableto, array $expected): void {
         $this->resetAfterTest();
 
         $course = $this->getDataGenerator()->create_course();