MDL-75077 output: prevent duplicate ID errors in custom menus.

Following on from 8e4a7c6b, we should apply a similar change to
custom menus to account for being collapsed into the "More" menu.

lib/outputcomponents.php

@@ -3657,6 +3657,7 @@ class custom_menu_item implements renderable, templatable {
         $syscontext = context_system::instance();
 
         $context = new stdClass();
+        $context->moremenuid = uniqid();
         $context->text = external_format_string($this->text, $syscontext->id);
         $context->url = $this->url ? $this->url->out() : null;
         // No need for the title if it's the same with text.

lib/tests/navigation/output/primary_test.php

@@ -159,7 +159,20 @@ class primary_test extends \advanced_testcase {
         $method = new ReflectionMethod('core\navigation\output\primary', 'get_custom_menu');
         $method->setAccessible(true);
         $renderer = $PAGE->get_renderer('core');
-        $this->assertEquals($expected, $method->invoke($output, $renderer));
+
+        // We can't assert the value of each menuitem "moremenuid" property (because it's random).
+        $custommenufilter = static function(array $custommenu) use (&$custommenufilter): void {
+            foreach ($custommenu as $menuitem) {
+                unset($menuitem->moremenuid);
+                // Recursively move through child items.
+                $custommenufilter($menuitem->children);
+            }
+        };
+
+        $actual = $method->invoke($output, $renderer);
+        $custommenufilter($actual);
+
+        $this->assertEquals($expected, $actual);
     }
 
     /**