Documents why the supervisor suite stays sequential (claim is
cross-tenant via LoadNextPendingForUpdateSkipLocked; parallel
supervisors would steal each other's runs) and switches the SIGTERM
subtest to errors.AsType[*exec.ExitError] to match the project
convention enforced elsewhere in this file.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>