Admin Bar shows "Add User" link to users without 'create_users' cap

[dlh]

On a single-site install, a user needs the create_users capability to add a user, but the "User" link still appears in the Admin Bar's "New" menu for users with promote_users. (The user is blocked from adding a user if they visit the link.)

At a glance, it looks like a similar issue was fixed in #30526, and the attached patch would take the same approach by checking is_multisite() along with current_user_can( 'promote_users' ) before adding the link.

[johnbillion]

Thanks for the patch, @Soean.

We should add a test for this in the Tests_AdminBar test class.

[dlh]

39252.2.diff​ adds some tests.

[johnbillion]

In 40581:

Users: Correct a permission check when showing the User item in the + New admin toolbar menu.

Props dlh
Fixes #39252

[johnbillion]

@dlh Thanks again for the patch. FYI I tweaked those tests a little so they use the get_standard_admin_bar() method instead of calling the admin menu bar population functions manually.