fix(r/derived_column): tidy up test

[jharley]

Noticed this test was using a fixed name during another investigation.

Tidies up the test a bit as well.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.78%. Comparing base (51eb48e) to head (6adbba7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #547 +/- ## ========================================== + Coverage 78.72% 78.78% +0.06% ========================================== Files 86 86 Lines 6124 6124 ========================================== + Hits 4821 4825 +4 + Misses 968 964 -4 Partials 335 335 ```

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