Switch from mockr to testthat for mocking

[georgestagg]

No description provided.

[jennybc]

In general, getting into position to fully run googledrive's tests locally is a big commitment.

But from a quick look, I've only used mocking in tests that don't require auth, i.e. tests of internal utilities that don't hit the Drive API. So it seems like this would be feasible for tidyverse dev day. If someone picks this up, you can expect to see lots of skipped tests, but the tests you're touching should run. And then once the PR is pushed, full tests will be run in CI.