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.

course/tests/course_format_function_test.php

@@ -37,7 +37,7 @@ class course_format_function_test extends \basic_testcase {
      * Tests clean_param_if_not_null function
      * @covers ::clean_param_if_not_null
      */
-    public function test_clean_param_if_not_null() {
+    public function test_clean_param_if_not_null(): void {
         $this->assertNull(clean_param_if_not_null(null));
         $n = '3x';
         $this->assertEquals(clean_param($n, PARAM_INT), clean_param_if_not_null($n, PARAM_INT));
@@ -53,7 +53,7 @@ class course_format_function_test extends \basic_testcase {
      * Tests contract_value function
      * @covers ::contract_value
      */
-    public function test_contract_value() {
+    public function test_contract_value(): void {
         $input = [
             'abc' => '<p>All together Now</p>',
             'abcformat' => '1',
@@ -80,7 +80,7 @@ class course_format_function_test extends \basic_testcase {
      * Tests expand_value function
      * @covers ::expand_value
      */
-    public function test_expand_value() {
+    public function test_expand_value(): void {
         $input = [
             'abc_editor' => [ 'text' => '<p>All together Now</p>', 'format' => '1' ],
             'jolly' => 'Roger',