tests: (lsfd::column-xmode) ignore "rwx" mappings

tests: (lsfd::column-xmode) ignore "rwx" mappings

This change fixes the issue about lsfd reported in
https://marc.info/?l=util-linux-ng&m=169972678725084&w=2

column-xmode test case assumes an executable file has only
one "x" mapping like:

    % pmap $$ | grep ${SHELL##*/}
    1292343:   zsh
    0000562386c1c000     92K r---- zsh
    0000562386c33000    716K r-x-- zsh <<
    0000562386ce6000     88K r---- zsh
    0000562386cfc000      8K r---- zsh
    0000562386cfe000     24K rw--- zsh

However, this assumption is wrong on Sparc64:

    % pmap $$
    1490470:   zsh
    0000010000000000    792K r-x-- zsh <<
    00000100001fe000      8K r---- zsh
    0000010000200000     32K rwx-- zsh <<

Two "x" mappings are found.
As a result, the original test case failed.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>