migrate unit tests from plt

[corybrunson]

Some students working with me recently exhaustively unit-tested {plt}, and i didn't think to tell them that the "persistence" class and associated functions would be deprecated once implemented in {phutil}. They prepared these unit tests for the class. Could we migrate them here? It seems like {tinytest} syntax is quite close to that of {testthat}, and at a glance it looks like they cover some behavior not covered by the existing tests.

[astamm]

Sure! Do you want to contribute a PR or would you want me to do it? Either way is fine by me.

[corybrunson]

They probably need to be adapted to the differences between the classes, so we'll do the adapting and submit a PR. Thanks!

[corybrunson]

This was done in #41.