Extend TestFileManager

Extend TestFileManager

A test fixture may wish to change the output temporary directory, and
should be able to do so without perturbing the global state from which
the test TestFileManager object is initialized.

The only client of this feature (the setup code) is updated
accordingly.

Minor fixes to existing documentation.

Change-Id: I7ba7a82964c05f12de107710655b6320d42ae99c