fix(tests): use streamOutput for test runs

This PR uses `bufio` to stream test output to stdout/stderr while also capturing it for assertions.  This will hopefully work around some of the buffering issues we've seen result in timeouts/SIGQUITs on `git push`, `deis run` and `deis pull`.  It also makes for nicer test output since the results are streamed live. :smile:
A [MultiWriter](http://golang.org/pkg/io/#MultiWriter) could prove useful here.