clean up tests/testthat/packrat library folder after running tests

... to avoid issues when running `R CMD build`; e.g.

```
Warning in utils::tar(filepath, pkgname, compression = compression, compression_level = 9L,  :
  storing paths of more than 100 bytes is not portable:
  ‘packrat/tests/testthat/packrat/lib-R/x86_64-apple-darwin17.0/4.0.3/spatial/tests/Examples/spatial-Ex.Rout.save’
```