With --progress, runCheck started the progress goroutine but waited
only for the results goroutine, so check could log its summary, or
return, before the last progress line was written and cleared. The
progress goroutine now signals a sync.WaitGroup when it finishes, and
runCheck waits on it as soon as Check returns, as generate does.
The new test sends stdout and stderr to one buffer and slows down
stdout writes, so without the wait the progress output lands after
the summary.
Model: opus-5-5
sneak