test_external_idp: update code for selenium 4.10
    
    The integration test is using selenium web driver to simulate a
    user authentication with an external IdP. The user performs kinit
    and is provided with a URL where he needs to authenticate.
    
    The test was written for selenium API 4.9 and must be adapted to
    the changes introduced in 4.10:
    - the headless method has been deprecated
    - executable_path argument has been deprecated
    
    Fixes: https://pagure.io/freeipa/issue/9493
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>