network-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/bin/network-test - from package: ifupdown-extra

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

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