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
Fix tests on Windows by wrapping baseline paths in file_path

* use internal file_path function to normalise baseline paths in the same
  way as the xxx_dir() functions
* using file_path avoids having to use:
  normalizePath(file.path(), mustWork=FALSE)
Merge pull request #10 from jefferis/fix/wintests

Fix tests on Windows