Add high-order derivatives test to test file for diff().
Add high-order derivatives test to test file for diff().

3715884c added code which checks to see if the differentiation order is higher
than the length of the chebtech being differentiated and automatically returns
the zero chebtech if so.  This test exercises that branch of code.

ilovejs/chebfun,alshedivat/chebfun,asgeirbirkis/chebfun