fmt
Make work with latest cmdtest

Latest cmdtest update [1] from vito changed OutputWrapper to take an io.Writer
rather than an io.Reader. This means the wrapper needs to use a MultiWriter
rather than a TeeReader to echo the output in verbose mode.

[1] https://github.com/vito/cmdtest/commit/638231df49972b83317352c72ab367f70bad4f6c