When running under rspec bisect runner, $stdout & $stderr are reassigned
to a StringIO, which would introduce spec errors.

("can't convert Tempfile into StringIO", when trying to reopen the
stream in CaptureStreamToTempfile)

Since CaptureStreamToTempfile reopens the stream rather than reassigning
it, I _think_ it makes sense to use STDOUT rather than whatever the
current value of $stdout is.

Otherwise it doesn't work when $stderr has previously been reassigned,
like in the rspec-core test suite