Add test-crontab-sorting script.

To activate for example in a pre-commit hook, use:

| # Test crontabs for sorting.
| git diff --cached --name-only --diff-filter=ACM -z $against |
| while read -rd '' FILENAME; do
|     case "$FILENAME" in
|         cronietab.*|crontab.*)
|             build-aux/test-crontab-sorting <(git show :"$FILENAME") || exit 1
|             ;;
|     esac
| done