ui_tests: extend test_selinuxusermap.py suite
    
    Extend test_selinuxusermap.py suite with new test cases. Details in
    the ticket.
    
    We also modify "add_table_associations" to handle "cancel" and
    "negative" in the way other methods works.
    
    Lastly, we start using dialog_btn=None to test keyboard confirmation
    as we did use it incorrectly with "Negative=True" where it was already
    confirmed by "click".
    
    Added tests:
    
    addselinuxusermap_MLS_singlelevel
    addselinuxusermap_cancel
    addselinuxusermap_disabledhbacrule
    addselinuxusermap_MLS_range
    addselinuxusermap_MCS_range
    addselinuxusermap_MCS_commas
    addselinuxusermap_MLS_singlevalue
    addselinuxusermap_multiple
    addandeditselinuxusermap
    selinuxusermap_undo
    selinuxusermap_refresh
    selinuxusermap_reset
    selinuxusermap_update
    selinuxusermap_backlink_cancel
    selinuxusermap_backlink_reset
    selinuxusermap_backlink_update
    selinuxusermap_deletemultiple
    add_user_selinuxusermap_cancel
    add_host_selinuxusermap_cancel
    add_hostgroup_selinuxusermap_cancel
    selinuxusermap_requiredfield
    selinuxusermap_duplicate
    selinuxusermap_nonexistinguser
    selinuxusermap_invalidusersyntaxMCS
    selinuxusermap_invalidusersyntaxMLS
    add_usernegative_selinuxusermap
    selinuxusermap_addNegativeHBACrule
    selinuxusermap_search
    selinuxusermap_searchnegative
    selinuxusermap_disablemultiple
    selinuxusermap_enablemultiple
    selinuxusermap_deleteNegativeHBACrule
    add_selinuxusermap_adder_dialog_bug910463
    delete_selinuxusermap_deleter_dialog_bug910463
    
    https://pagure.io/freeipa/issue/7544
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>