Structure of tests

[mhesselbarth]

At some point, we should revise all tests and not only check if the returning objects are tibbles (as done now), but also check if the content makes sense (e.g. if the correct types/levels of metrics are present)

[mhesselbarth]

I worked a bit on the tests today and there seem to be some weird warnings if you use the "Test package" shortcut of RStudio. The warnings are not present if you just execute the functions on the console.

[mhesselbarth]

Okay...I fixed the problem with the local warnings in 05c91f0. The problem was that during the checks the global option to_disk = TRUE was set (and not reset) which followed to some warnings afterward.