Clarify TestFileManager naming and comments.

Clarify TestFileManager naming and comments.

- Rename datapath.* to testfilemanger.*, since they now only declare
  that one class.
- Make functions that return input data paths have "Input" explicitly in
  their name to make it clear what data path they are returning.  Also
  remove redundant "Test" from the names and replace "Path" by
  "Directory" for the functions that deal with directories.
- Some Doxygen documentation updates.

Change-Id: If80b5d83d075f3ce29b7b40c0d17f319166332b8