Investigate testthat::{with,local}_mocked_bindings

[brookslogan]

We might want to take a look at testthat::{with,local}_mocked_bindings, which takes lessons from mockr and mockery. (This is not to be confused with the deprecated testthat::with_mock.)

Issues with status quo: while testing #99 I encountered some issues with mockery::stub where (a) a natural-seeming stub call wasn't actually correct, and (b) adjusting to try to use depth didn't seem to work, though a workaround was easy, and (c) when attempting to debug some tests, I wanted to evaluate some chunks of the test in a REPL, but running stub has lasting effects unless packaged inside a test_that or local({ <code> }).