ipatests: increase test_webui_server timeout
    
    test_webui_server tends to take more than 3600s to run.
    Increase timeout to 7200s.
    
    Fixes: https://pagure.io/freeipa/issue/8266
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Armando Neto <abiagion@redhat.com>