Add "consume all captured output" callback to CaptureFD

Add "consume all captured output" callback to CaptureFD

Summary: I noticed myself trying to fake this kind of callback for a log-based test I was writing. It seems much nicer to add the callback to `CaptureFD` than roll ugly wrappers around it to do the same thing.

Reviewed By: @yfeldblum

Differential Revision: D2506106