This is normal rule to return an error from a task if the task has to
abort due to cancellation. We already do this in tee, but not in the
function that is waiting for spawned process to complete(*).

-> Fix that. Wrap corresponding wg.wait() into try/except and check if
it fails due to cancellation, upon which we should not raise, but
instead should continue to finish current test_result_line and only
after stop test run normally (again without raise, but with regular one
line log entry).

(*) added in 0ad45a9c (Detect if a test leaks processes and terminate them)

/reviewed-by @jerome
/reviewed-on nexedi/nxdtest!14