Unify Repro CI Experiment Checksum structure with Config Repo structure

[CodeGat]

The Problem

Currently, the struture of the checksums in a repro test experiment running on Gadi are the following:

$EXPERIMENT_LOCATION/
└── checksum/
    ├── historical-*hr-checksum.json
    └── test_report.xml

And the structure of the checksums in the repo are:

.
└── testing/
    └── checksum/
        ├── historical-*hr-checksum.json
        └── test_report.xml

In which, in the CI, we special-case the logic for creating the intermediate testing directory in the repo. This also creates problems for the inputs.additional-artifact-content-paths, which are, confusingly, in the correct place at both the experiment level and the repo level, which means they both have to be handled differently.

The Solution

Unify the checksum directories so that both the experiment and repo reflect the following structure:

$EXPERIMENT_LOCATION_OR_REPO_ROOT/
└── testing/
    └── checksum/
        ├── historical-*hr-checksum.json
        └── test_report.xml