Make helpers work with latest cmdtest

Recent vito/cmdtest update \[1] (from about a month ago) changed cmdtest.OutputWrapper to 
take an io.Writer rather than an io.Reader. This means that the latest cf-acceptance-tests
don't compile \[2] with the latest cmdtest version. 

This change updates teeStderr/out to use a MultiWriter rather than a TeeReader 
which makes go happy, and still prints the output properly in verbose mode.

[1] vito/cmdtest@638231d
[2] "/run.go:16: cannot use teeStdout (type func(io.Reader) io.Reader) as type cmdtest.OutputWrapper in function argument"