Implement unit tests with at least 50% coverage

[andysontran]

• Use testthat package to implement unit tests for sawmill package, aiming for 50% test coverage by end of 2024-06

• Create and store tests in tests/testthat/test-function name.R (automatically generated by usethis::use_test("function name"); refer to https://r-pkgs.org/testing-basics.html#create-a-test)

• Use snapshot tests to record expected results (refer to https://r-pkgs.org/testing-basics.html#sec-snapshot-tests)

• Check test coverage using covr package (refer to https://covr.r-lib.org/)

[andysontran]

Using the diffviewer package by calling testthat::snapshot_review('read_timber/') following a failed test, we can view the changes in .CSV output for the respective function being tested.