Expand test coverage

[karthik]

trafficstars

We need to spend a bit of time improving coverage. Some bits will be challenging/impossible (e.g. auth) but we should be able to make improvements elsewhere

[ClaytonJY]

Should we have a specific target? 80%? 90%?

I think I can do a fair bit more with auth than I am currently; something like https://github.com/jennybc/googlesheets/blob/master/tests/testthat/test-auth.R

[karthik]

Leaving a note here to expand coverage for 2nd, + nth level folder issues similar to #142