Move tests to testthat's new preferred location for CRAN compatibility

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.