solenv: fix bashism in unittest-failed.sh

Change-Id: If030cd711cbb04fd622a4fd5723b6c66aac28a98
Reviewed-on: https://gerrit.libreoffice.org/13546
Reviewed-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Tested-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>