Convert documentation examples to tests

I started converting a bunch of the examples shown in the documentation to tests; for example see commits 8025fe78ba15395cd07a0a410c0240c8e2e11282 and be3dbb11f58d2d79fe702aa40bc033163f25537c. It would be great if someone could help converting over more of them! As nice as it would be, I don't think it's feasible to use something like stoeffel/elm-verify-examples since most tests in this package require using custom comparison functions that include a small numerical tolerance for roundoff.

The general procedure would be to find examples in a particular module, convert them to tests in the corresponding tests file, then run the tests locally using elm-test to verify that they pass.