isotester 5-8 don't reboot before running test suite jobs

Originally created by @intrigeri on #11467 (Redmine)

… and this breaks tests, e.g. because chutney can’t start all its Tor processes. It seems that each attempt to reboot fails due to:

May 19 11:52:12 isotester7 sshd[3891]: Connection closed by 192.168.122.11 [preauth]

(192.168.122.11 is jenkins.lizard)