[#20586] Ensure the assumed "setup" dir is relative to the harness, not CWD

Prior to this the harness assumed that a number of tasks were in it's
setup folder but it loaded them relative to the CWD. This causes issues
when trying to run the harness as a gem executable outside of the
traditional directory structure. This commit fixes that so that all
assumptions around the location of the 'setup' dir is relative to the
harness's root not the CWD.