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.

competency/tests/course_module_competency_test.php

@@ -25,7 +25,7 @@ namespace core_competency;
  */
 class course_module_competency_test extends \advanced_testcase {
 
-    public function test_count_competencies() {
+    public function test_count_competencies(): void {
         global $CFG, $DB;
 
         $this->resetAfterTest(true);