testrb: 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 CentOS possible default full paths are:

/usr/bin/testrb - from package: ruby

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/testrb - from package: ruby
/usr/lib/jruby/bin/testrb - from package: jruby
/usr/lib/ruby/gems/2.1.0/gems/test-unit-2.1.5.0/bin/testrb - from package: libruby2.1
/usr/lib/ruby-standalone/bin/testrb - from package: ruby-standalone

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

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