Duplicated test run and report

[famz]

With the current test dispatching logic, two or more testers can get the same task and both report success/failure. Upon failure, two events are generated, as a result, two email reports can be sent:

https://lists.gnu.org/archive/html/qemu-devel/2018-03/msg00279.html

[bonzini]

Should be fixed, or at least mitigated, by commit 0a17dcd ("testing: do not restart jobs too soon").