[R] Set up testthat infrastructure and run.py r_unit_test

[Amasus]

Part of #235.

Create the testthat skeleton — no tests yet, just the scaffolding:

• Create `R/tests/testthat/` directory
• Add a `testthat.R` runner script
• Add a shared `helper.R` with common fixture-building utilities (small data.tables representing config, result, and EDE data matching the column schemas used by the utility functions)
• Add `python run.py r_unit_test` command that runs `Rscript -e "testthat::test_dir('R/tests/testthat/')"` inside the container

This is a prerequisite for all subsequent test phases.