Menus: Add menu-$i slugs to mapping groups

Helps to future proof the feature.
Also orders slugs by popularity to optimize mapping time.

See #39692.


git-svn-id: https://develop.svn.wordpress.org/trunk@42026 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/menu/nav-menu.php

@@ -172,8 +172,8 @@ class Tests_Nav_Menu_Theme_Change extends WP_UnitTestCase {
 		$new_next_theme_nav_menu_locations = wp_map_nav_menu_locations( $old_next_theme_nav_menu_locations, $prev_theme_nav_menu_locations );
 
 		$expected_nav_menu_locations = array(
-			'main' => 1,
-			'primary' => 2,
+			'primary' => 1,
+			'main' => 2,
 		);
 		$this->assertEquals( $expected_nav_menu_locations, $new_next_theme_nav_menu_locations );
 	}