Test jobs sometimes get scheduled on a busy isotester while there are available ones

Originally created by @bertagaz on #11295 (Redmine)

While investigating #10601 (closed), we discovered that sometimes after a reboot_job completed, rather than starting the test job that triggered it for this isotester, Jenkins assigns this same isotester to another test job, resulting in the first test job waiting for hours for the other one to be over. See #10601 (comment 68441) for details.

Related issues

Edited by bertagaz