Always stream test output concurrently

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 e5a070135222a53a6ab753f98d724aaf25a61a35)