tests: fix spurious failures due to dpkg install-info

tests: fix spurious failures due to dpkg install-info

* t/install-info-dir.sh: Skip some checks if 'install-info' is the one
from dpkg, not the one from GNU info; the former might try to create
files in '/var/backups/', causing spurious failures like this for any
non-root user:

    install-info(.../install-info-dir.dir/_inst/info/foo.info): \
      creating new section `Dummy utilities'
    cp: cannot create regular file `/var/backups/infodir.bak': \
      Permission denied
    install-info(.../install-info-dir.dir/_inst/info/foo.info): \
      could not backup .../install-info-dir.dir/_inst/info/dir in \
      /var/backups/infodir.bak: No such file or directory

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>