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/external/tests/external_value_test.php

@@ -54,7 +54,7 @@ class external_value_test extends advanced_testcase {
      * @param mixed $requiredstr The string value of the $required param in the debugging message.
      * @return void
      */
-    public function test_required_param_validation($required, $debuggingexpected, $requiredstr = '') {
+    public function test_required_param_validation($required, $debuggingexpected, $requiredstr = ''): void {
         $externalvalue = new external_value(PARAM_INT, 'Cool description', $required);
         if ($debuggingexpected) {
             $this->assertDebuggingCalled("Invalid \$required parameter value: '{$requiredstr}' .