Test location

testthat now prefers to have tests stored in ``tests/testthat``, rather than ``inst/tests`` so that the user has control over whether or not tests are installed using the ``--install-tests parameter`` to ``R CMD install``, or ``INSTALL_opts = c("--install-tests")`` argument to ``install.packages()``. This moves the tests to the new, preferred location, along with the test data (in ``tests/testthat/testdata``). In addition, all examples that rely on the presence of the test data are wrapped in ``\dontrun{}`` as there is no longer any guarantee that this data will be available.