Add prefix path option for test config in reporter

[islas]

The current rel_offset only captures command-line input necessary for tests to run correctly, but not the path to the test config.

Since the root directory always starts from the test config, then applies the command-line directory offset if requested there is no runtime metadata regularly available on where the test config may be located within a project. Similar to the --exec relative location of the runner.py we can provide the relative prefix of the test config from the project root directory.