Convert remaining standalone tests to testthat framework (#138)

* Initial plan

Co-authored-by: krivit <15682462+krivit@users.noreply.github.com>

* Convert remaining tests to testthat framework

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Co-authored-by: krivit <15682462+krivit@users.noreply.github.com>

* Address review: add explanatory comments in test-target-offset.R

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Co-authored-by: krivit <15682462+krivit@users.noreply.github.com>

* style: reformat 8 test files to Tidyverse Style Guide

- Add spaces around assignment (<-), binary operators, and = in function args
- Replace single quotes with double quotes for strings
- Replace T/F with TRUE/FALSE
- Break long lines (>80 chars) with each argument on its own line
- Add space after # in comments
- Use 2-space indentation consistently
- Wrap long formulas and function calls

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: krivit <15682462+krivit@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: krivit <15682462+krivit@users.noreply.github.com>

Author: Copilot