Make some unit tests use mock file output

Make some unit tests use mock file output

Introduce TextOutputStream interface that can be used instead of direct
file output.  Make unit tests for help output use an in-memory stream
instead of a real file using this mechanism.

TextWriter wraps the raw stream and provides more convenient
line-oriented writing capabilities (and provides a natural place to
implement more of the same, making them immediately available
independent of the stream used).

Most of the touched files only contain mechanical
  File -> TextWriter/TextOutputStream
and related documentation replacements.  The main non-trivial changes
are introduction of the streams and TextWriter in utility/, and
reorganization of the way the tests use TestOutputRedirector.

Further work follows to allow full removal of the old File class and to
extend the use of the streams.

Change-Id: I7700d0d8af5f44b304b940797a4834993e4738fb