[R] Test result data transformation functions

[Amasus]

Part of #235. Depends on #237.

Tests for functions that transform solver output data in `graph_functions.R` and `regression_functions.R`.

A minimal fixture result data.table should be added to `helper.R` (from #237) matching the column schema of the `residence_distances`, `edes`, and `precinct_distances` output files.

Functions to cover:

• `set_results_column_names()` — column present, column absent (no-op)
• `ede_with_pop()` — correct population scaling calculation
• `demographically_weighted_distances()` — weighted distance computation
• `bg_data()` — density and percentage columns added correctly; NULL input with historic flag returns NULL