Simplify testing code with fixtures

[skent259]

See https://r-pkgs.org/test-design.html

• Want to wrap all data into fixtures
• Remove calls to "library"
• Use skip package function in test that instead of my own helpers
• Re-name test files with a "-" instead of "_"

[skent259]

Progress made with #65. Remains to wrap data into fixtures for some test files