Adding more tests

[jzwart]

• Adds 8 new test files and updates 1 existing test file covering gas exchange models,
  oxygen saturation, Schmidt numbers, PAR conversions, wind scaling, metabolism models, and
  helper functions (170 tests total)
• Restricts test coverage CI workflow to Ubuntu only

Note:

• devtools::test() passes with 0 failures and 0 warnings
• devtools::check(args = "--as-cran") passes cleanly