brightness_test: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/lib/plainbox-providers-1/checkbox/bin/brightness_test - from package: plainbox-provider-checkbox

You may install the required package with command apt-get install {package}

Error in other languages:
brightness_test: bevel nie gevind nie
brightness_test: comandă negăsită
brightness_test: comando não encontrado
brightness_test: commande introuvable
brightness_test: command not found
brightness_test: käsku ei ole
brightness_test: không tìm thấy lệnh
brightness_test: komanda nerasta
brightness_test: Kommando nicht gefunden.
brightness_test: kommandot finns inte
brightness_test: komut yok
brightness_test: nie znaleziono polecenia
brightness_test: níor aimsíodh an t-ordú
brightness_test: no se encontró la orden
brightness_test: no s'ha trobat l'ordre
brightness_test: opdracht niet gevonden
brightness_test: parancs nem található
brightness_test: perintah tidak ditemukan
brightness_test: príkaz nenájdený
brightness_test: příkaz nenalezen
brightness_test: команда не найдена
brightness_test: командата не е открита
brightness_test: 命令找不到
brightness_test: コマンドが見つかりません