promote logger and writer mock objects from pathfinder utils

[bob-carpenter]

Summary:

There are some generally useful testing classes in src/test/unit/services/pathfinder/util.hpp that'd be useful higher up. At that point, they should be renamed to something like mock_writer and mock_logger.

Description:

Also add doc.

Current Version:

v2.32.2

[kaifkh20]

Is this issue still open?

[mitzimorris]

yes it is still open.

it looks like this is your first time contributing to Stan. a few requests

• please read the Stan developer process and guidelines, start here: https://github.com/stan-dev/stan/wiki/Introduction-to-Stan-for-New-Developers
• please ask questions! especially of the Stan devs who worked on Add Pathfinder and Multi Pathfinder #3123 in order to understand what they're looking for here.