unattend.inf only allows to add RunOnce entries, which are in theory only executed on the first boot after installation.
For almost a decade, this worked anyway to start regtest.cmd again after a ReactOS crash, because the registry and thus the deletion of the RunOnce entry wasn't stored on disk in case of a crash.
Thanks to Sylvain for the hint!
As Explorer processes the Run key right after RunOnce and therefore picks up regtest.cmd a second time during the first boot, exit the script just after adding the Run key.
Otherwise, rosautotest.exe would be started twice, with catastrophic results.