ttuegel authored May 20, 2014 and tibbe committed May 20, 2014

Issue #1810. Some test suites would freeze if invoked with
`--show-details=always` instead of `--show-details=streaming` because
output would build up in the pipe without being cleared. This corrects
the issue by forcing the length of the output string in another thread.

(cherry picked from commit e5a07013)