Switch tests to tests/testthat

* this is the new recommended location, allowing tests to be installed or not
* see https://github.com/hadley/testthat#integration-with-r-cmd-check
make all tests os dependent if os arg was used

* need to add some more windows tests
* this means that multi platform travis builds and winbuilder will be more
  important than ever
remove os argument from examples
deactivate another test on windows

* parse_path_string is only called from file_path_site_unix anyway
use file_path not file.path in Windows tests

* file_path calls normalizePath which results in backwards slash paths on
  Windows, so we need the same in our tests.
* which are becoming rather close to reimplementing the function ...